Make it possible to assign clang-format-buffer to a keybinding.

Patch by Chris Gray.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183739 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/clang-format/clang-format.el b/tools/clang-format/clang-format.el
index 48fe36f..b0131ed 100644
--- a/tools/clang-format/clang-format.el
+++ b/tools/clang-format/clang-format.el
@@ -28,6 +28,7 @@
 
 (defun clang-format-buffer ()
   "Use clang-format to format the current buffer."
+  (interactive)
   (clang-format (point-min) (point-max)))
 
 (defun clang-format (begin end)