scripts: release: do not complain if release dir exists

I just don't like the error message when building tar files.

Signed-off-by: Eric Paris <eparis@redhat.com>
diff --git a/scripts/release b/scripts/release
index 88fa9c8..80c1653 100755
--- a/scripts/release
+++ b/scripts/release
@@ -2,7 +2,7 @@
 
 DIRS="libsepol libselinux libsemanage policycoreutils checkpolicy sepolgen"
 
-mkdir ../selinux-dev-release
+mkdir -p ../selinux-dev-release
 
 for i in $DIRS; do
 	cd $i