Make audio_extn build with the VNDK.

When the vndk is enabled (BOARD_VNDK_VERSION=current),
vendor libraries must only link against vendor variants
in the build system. This also means that they receive
restricted sets of headers and that automatic global
headers are restricted.

Test: build audio_extn with BOARD_VNDK_VERSION=current
Bug: 69574580
Change-Id: Iaae465a68d04bb991a95442a0114591849d282a3
diff --git a/hal/audio_extn/a2dp.c b/hal/audio_extn/a2dp.c
index 516383a..1a9421b 100644
--- a/hal/audio_extn/a2dp.c
+++ b/hal/audio_extn/a2dp.c
@@ -20,6 +20,7 @@
 
 #include <dlfcn.h>
 #include <errno.h>
+#include <pthread.h>
 #include <stdlib.h>
 
 #include <cutils/log.h>