Add compile_commands.json to gitignore.

This is used for YouCompleteMe completions.

Change-Id: I72b56b82b5fa8d6aff6d1749eca82aad042fa513
diff --git a/.gitignore b/.gitignore
index 881f4d3..3d451b8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,3 +15,4 @@
 /out*
 TAGS
 /node_modules/
+/compile_commands.json