.gitignore: add compile_commands.json

Add Clang's compilation database file (i.e. compile_commands.json) to
.gitignore, at the root of the repository.

Signed-off-by: Joao Marcos Costa <jmcosta944@gmail.com>
Tested-by: Joao Paulo Goncalves <joao.goncalves@toradex.com>
diff --git a/.gitignore b/.gitignore
index aa01604..84051b7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -109,3 +109,6 @@
 
 # qconfig database
 /qconfig.db
+
+# Clang's compilation database file
+/compile_commands.json