genbuild_h: Add Mac OS to case of "data -r"

Change-Id: I2a43a1b1749da207b7a23b8eb252c13605121533
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/11599
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

util/genbuild_h/genbuild_h.sh

@@ -40,7 +40,7 @@ fi
 
 our_date() {
 case $(uname) in
-NetBSD|OpenBSD|DragonFly|FreeBSD)
+NetBSD|OpenBSD|DragonFly|FreeBSD|Darwin)
 	date -r $1 $2
 	;;
 *)