Ignore .gdb_history files

Summary: I tend to accumulate these and they are often quite useful to keep around.

Reviewers: monoidal

Reviewed By: monoidal

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5320

.gitignore

@@ -215,3 +215,4 @@ GIT_COMMIT_ID
 ghc.nix/
 
 
+.gdb_history
\ No newline at end of file