.gitignore: add GNU GLOBAL files

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/.gitignore b/.gitignore
index ed21203..4116448 100644
--- a/.gitignore
+++ b/.gitignore
@@ -79,5 +79,11 @@
 /ctags
 /etags
 
+# gnu global files
+GPATH
+GRTAGS
+GSYMS
+GTAGS
+
 # spl ais files
 /spl/*.ais