Fix an error that occurs with Meson 0.50.0 and later:
"ERROR: Subdir keyword must not be an absolute path".

--- a/src/meson.build	2019-03-15 00:10:26.882293850 +0100
+++ b/src/meson.build	2019-03-15 00:10:54.312197229 +0100
@@ -82,7 +82,7 @@
 
 install_headers(
   headers,
-  subdir: join_paths(totem_includedir, totem_api_path)
+  install_dir: join_paths(totem_includedir, totem_api_path)
 )
 
 libtotem_player_sources = files(
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