.gitignore: Add packaged staging directory to ignore list

Signed-off-by: Joshua Lock <josh@linux.intel.com>

.gitignore

@@ -3,6 +3,7 @@
 build/conf/local.conf
 build/conf/bblayers.conf
 build/tmp/
+pstage/
 scripts/poky-git-proxy-socks
 sources/
 meta-darwin