gitignore: Add GNU GLOBAL files to top .gitignore

Ignore GPATH, GRTAGS, GSYMS, and GTAGS generated by GNU GLOBAL.
Signed-off-by: NJani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

.gitignore

@@ -63,6 +63,12 @@ series
 cscope.*
 ncscope.*
 
+# gnu global files
+GPATH
+GRTAGS
+GSYMS
+GTAGS
+
 *.orig
 *~
 \#*#