Fix build with FreeType 2.10.3 and newer.

Taken from upstream:
https://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff;h=41ef9a0bc36b9db7115fbe9623f989bfb47bbade

diff --git a/base/fapi_ft.c b/base/fapi_ft.c
--- a/base/fapi_ft.c
+++ b/base/fapi_ft.c
@@ -125,7 +125,7 @@ static void
 delete_inc_int_info(gs_fapi_server * a_server,
                     FT_IncrementalRec * a_inc_int_info);
 
-FT_CALLBACK_DEF(void *)
+static void *
 FF_alloc(FT_Memory memory, long size)
 {
     gs_memory_t *mem = (gs_memory_t *) memory->user;
@@ -133,7 +133,7 @@ FF_alloc(FT_Memory memory, long size)
     return (gs_malloc(mem, size, 1, "FF_alloc"));
 }
 
-FT_CALLBACK_DEF(void *)
+static void *
     FF_realloc(FT_Memory memory, long cur_size, long new_size, void *block)
 {
     gs_memory_t *mem = (gs_memory_t *) memory->user;
@@ -153,7 +153,7 @@ FT_CALLBACK_DEF(void *)
     return (tmp);
 }
 
-FT_CALLBACK_DEF(void)
+static void
     FF_free(FT_Memory memory, void *block)
 {
     gs_memory_t *mem = (gs_memory_t *) memory->user;
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<tr><td><span title='2019-12-26 12:00:01 +0100'>2019-12-26</span></td><td><a href='/guix/commit/gnu/packages/axoloti.scm?id=dedc66ef8ecc91fe753f1e6c23102bfa9d79a808'>gnu: axoloti-patcher-next: Remove commented expression.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2019-12-25 11:06:10 +0100'>2019-12-25</span></td><td><a href='/guix/commit/gnu/packages/axoloti.scm?id=5261553a084a8c216aa2e94f3cd62cd3b33ff56c'>gnu: Add axoloti-patcher-next.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2019-12-25 11:06:10 +0100'>2019-12-25</span></td><td><a href='/guix/commit/gnu/packages/axoloti.scm?id=ea38d2eea28e6c3151747eb292ffe9ccf747070b'>gnu: axoloti-patcher, axoloti-runtime: Update to 1.0.12-2.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2019-10-05 22:23:42 +0200'>2019-10-05</span></td><td><a href='/guix/commit/gnu/packages/axoloti.scm?id=07c1a10ef6cd24a0a02aefb281931fdd6fa3895e'>gnu: axoloti-runtime: Don't use unstable tarball.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>