ignore .gdb_history files

When using gdb, history files will often get generated.  So ignore them.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

.gitignore

@@ -47,6 +47,9 @@ patches-*
 patches
 series
 
+# gdb files
+.gdb_history
+
 # cscope files
 cscope.*