gralloc: Mark buffers used by HWC

Some clients need to distinguish between buffers going to HWC
for composition and other regular buffers.

Change-Id: Ia05a765afe2f8bd223f3e47edd0fb746d6ade531
diff --git a/libgralloc/gralloc_priv.h b/libgralloc/gralloc_priv.h
index c382a90..eefda21 100644
--- a/libgralloc/gralloc_priv.h
+++ b/libgralloc/gralloc_priv.h
@@ -157,6 +157,7 @@
             PRIV_FLAGS_VIDEO_ENCODER      = 0x00010000,
             PRIV_FLAGS_CAMERA_WRITE       = 0x00020000,
             PRIV_FLAGS_CAMERA_READ        = 0x00040000,
+            PRIV_FLAGS_HW_COMPOSER        = 0x00080000,
         };
 
         // file-descriptors