Make presubmit actually fail the build (#2013)

The exit code wasn't properly acted upon in the makefile.
Make filename-hyphen actually return an non-zero exit code.

Signed-off-by: Miek Gieben <miek@miek.nl>

.presubmit/filename-hyphen

@@ -5,5 +5,6 @@ echo "** presubmit/$(basename $0)"
 for dir in "$@"; do
     if find $dir | grep '-'; then
         echo "** presubmit/$(basename $0): please use an underscore in filenames instead of a hyphen"
+        exit 1
     fi
 done