Silence -Wunguarded-availability

The build system recently started to use `-Wunguarded-availability` and
is complaining that the calls to clock_gettime isn't properly guarded
with the runtime __builtin_availability check. However, the call is
already statically guarded by HAVE_CLOCK_GETTIME. Silence the unguarded
availability check.

Bug: 178124881
Test: watch a forrest run on mac
Change-Id: I0715091620ce1a5346f21c3484f4714d760bea61
diff --git a/Android.bp b/Android.bp
index 8a0ef0c..f293257 100644
--- a/Android.bp
+++ b/Android.bp
@@ -61,7 +61,8 @@
             cflags: [
-                "-Wno-deprecated-declarations"
+                "-Wno-deprecated-declarations",
+                "-Wno-unguarded-availability",