Mask out .cmake files

BUG=None
TEST=Run build_image and look at rootfs for any files left

Change-Id: I4983420fb17aae393c89b78ac93f655190241ee9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2681337
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: François Degros <fdegros@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
diff --git a/common.sh b/common.sh
index 2f7a535..27978b0 100644
--- a/common.sh
+++ b/common.sh
@@ -381,6 +381,7 @@
   *.a
   *.c
   *.cc
+  *.cmake
   *.go
   *.la
   *.h