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

diff --git a/.gitignore b/.gitignore
index 9c53f5c..e13fc96 100644
--- a/.gitignore
+++ b/.gitignore
@@ -47,6 +47,9 @@
 patches
 series
 
+# gdb files
+.gdb_history
+
 # cscope files
 cscope.*