gitignore: Add GNU Global tag files

This commit adds GNU Global[1] tag files to .gitignore.

[1]: https://www.gnu.org/software/global/

Change-Id: Ia09fb359cfdfeadd9538cf6352d353e6475e85c7
Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6541
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

.gitignore

@@ -95,3 +95,8 @@ TAGS
 
 # ctags tag files
 tags
+
+# GNU Global tag files
+GPATH
+GRTAGS
+GTAGS