Unbreak the open-source build for ARM due to missing ATTRIBUTE_PACKED
declaration.

snappy-stubs-internal.h

diff --git a/snappy-stubs-internal.h b/snappy-stubs-internal.h
index 9b2c11f..e51b731 100644
--- a/snappy-stubs-internal.h
+++ b/snappy-stubs-internal.h
@@ -140,6 +140,12 @@
       !defined(__ARM_ARCH_6ZK__) && \
       !defined(__ARM_ARCH_6T2__)
 
+#if __GNUC__
+#define ATTRIBUTE_PACKED __attribute__((__packed__))
+#else
+#define ATTRIBUTE_PACKED
+#endif
+
 namespace base {
 namespace internal {