don't silence libav test build output

for #496

Change-Id: I815cd4da17c079533faf691d707d5d1e5b551172

lib/common.Makefile

@@ -38,7 +38,7 @@ fix_frame_channel_layout.h:	../lib/fix_frame_channel_layout-*
 		rm -f "$@"; \
 		echo '/******** GENERATED FILE ********/' > "$@"; \
 		cat "$$x" >> "$@"; \
-		$(MAKE) fix_frame_channel_layout-test.o 2> /dev/null && break; \
+		$(MAKE) fix_frame_channel_layout-test.o && break; \
 		echo "Failed build with $$x"; \
 		rm -f "$@"; \
 	done; \