Use monotonically increasing clock for A2DP timing.

This is to avoid skips when the system time changes.
diff --git a/audio/liba2dp.c b/audio/liba2dp.c
index 5f069e5..9dfcf8b 100755
--- a/audio/liba2dp.c
+++ b/audio/liba2dp.c
@@ -153,9 +153,9 @@
 
 static uint64_t get_microseconds()
 {
-	struct timeval now;
-	gettimeofday(&now, NULL);
-	return (now.tv_sec * 1000000UL + now.tv_usec);
+	struct timespec now;
+	clock_gettime(CLOCK_MONOTONIC, &now);
+	return (now.tv_sec * 1000000UL + now.tv_nsec / 1000UL);
 }
 
 #ifdef ENABLE_TIMING