Fix memory leak when calling endpoint to select capabilities

When passing allocated memory to avdtp_service_cap_new it needs to be
freed since avdtp_service_cap_new copies the memory.
diff --git a/audio/a2dp.c b/audio/a2dp.c
index c5da708..c1d82bb 100644
--- a/audio/a2dp.c
+++ b/audio/a2dp.c
@@ -1833,6 +1833,7 @@
 						sizeof(*cap) + size);
 
 	setup->caps = g_slist_append(setup->caps, media_codec);
+	g_free(cap);
 
 done:
 	finalize_select(setup);