Fixes 'multiple definition errors' when building with GCC 10+

From: https://bugs.gentoo.org/710796
Originally from: https://gitlab.freedesktop.org/pipewire/pipewire/-/commit/a62e41e
and https://gitlab.freedesktop.org/pipewire/pipewire/-/commit/7a976c7

--- a/spa/plugins/bluez5/a2dp-codecs.h
+++ b/spa/plugins/bluez5/a2dp-codecs.h
@@ -284,15 +284,15 @@
         }
 }
 
-const a2dp_sbc_t bluez_a2dp_sbc;
+extern const a2dp_sbc_t bluez_a2dp_sbc;
 #if ENABLE_MP3
-const a2dp_mpeg_t bluez_a2dp_mpeg;
+extern const a2dp_mpeg_t bluez_a2dp_mpeg;
 #endif
 #if ENABLE_AAC
-const a2dp_aac_t bluez_a2dp_aac;
+extern const a2dp_aac_t bluez_a2dp_aac;
 #endif
 #if ENABLE_APTX
-const a2dp_aptx_t bluez_a2dp_aptx;
+extern const a2dp_aptx_t bluez_a2dp_aptx;
 #endif
 
 #endif
--- a/spa/plugins/bluez5/bluez5-monitor.c
+++ b/spa/plugins/bluez5/bluez5-monitor.c
@@ -75,7 +75,7 @@
 	struct spa_list transport_list;
 };
 
-struct spa_handle_factory spa_a2dp_sink_factory;
+extern struct spa_handle_factory spa_a2dp_sink_factory;
 
 static void fill_item(struct spa_bt_monitor *this, struct spa_bt_transport *transport,
 		struct spa_pod **result, struct spa_pod_builder *builder)
--- a/spa/plugins/bluez5/meson.build
+++ b/spa/plugins/bluez5/meson.build
@@ -1,7 +1,8 @@
 
 bluez5_sources = ['plugin.c',
 		  'a2dp-sink.c',
-                  'bluez5-monitor.c']
+		  'a2dp-codecs.c',
+		  'bluez5-monitor.c']
 
 bluez5lib = shared_library('spa-bluez5',
 	bluez5_sources,
ages/aux-files/linux-libre/5.12-i686.conf?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142'>commit</a><a href='/guix/diff/gnu/packages/aux-files/linux-libre/5.12-i686.conf?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/aux-files/linux-libre/5.12-i686.conf'>
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<div class='path'>path: <a href='/guix/log/?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142'>root</a>/<a href='/guix/log/gnu?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142'>gnu</a>/<a href='/guix/log/gnu/packages?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142'>packages</a>/<a href='/guix/log/gnu/packages/aux-files?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142'>aux-files</a>/<a href='/guix/log/gnu/packages/aux-files/linux-libre?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142'>linux-libre</a>/<a href='/guix/log/gnu/packages/aux-files/linux-libre/5.12-i686.conf?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142'>5.12-i686.conf</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/aux-files/linux-libre/5.12-i686.conf?id=2c5bba139237bb19ab4061452eeb3e7fdcdb1142&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-05-08 18:41:45 -0400'>2021-05-08</span></td><td><a href='/guix/commit/gnu/packages/aux-files/linux-libre/5.12-i686.conf?id=a21355bd1008d9f71dfb8ec6f4fdb33cf3a5007f'>gnu: Add linux-libre 5.12.2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/linux.scm (linux-libre-5.12-version, deblob-scripts-5.12,
linux-libre-5.12-pristine-source, linux-libre-5.12-source,
linux-libre-headers-5.12, linux-libre-5.12): New variables.
* gnu/packages/aux-files/linux-libre/5.12-arm.conf,
gnu/packages/aux-files/linux-libre/5.12-arm64.conf,
gnu/packages/aux-files/linux-libre/5.12-i686.conf,
gnu/packages/aux-files/linux-libre/5.12-x86_64.conf: New files.
* Makefile.am (AUX_FILES): Add them.
</span></span></td><td>Leo Famulari</td></tr>