mmc-utils: update to _FORTIFY_SOURCE=3

Since this project explicitly specifies its FORTIFY level, update it to
the level CrOS wants to move to.

BUG=b:288411201
TEST=CQ+1

Change-Id: Icba3f1175f3c6bc3d4315e55c012131bbf5037cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/mmc-utils/+/4707933
Tested-by: George Burgess <gbiv@chromium.org>
Reviewed-by: Sarthak Kukreti <sarthakkukreti@google.com>
Commit-Queue: George Burgess <gbiv@chromium.org>
diff --git a/Makefile b/Makefile
index aa27ff2..968524b 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 CC ?= gcc
-AM_CFLAGS = -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2
+AM_CFLAGS = -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=3
 CFLAGS ?= -g -O2
 objects = \
 	mmc.o \