gitignore: ignore .build-tools

The .build-tools is created by the scripts/build.sh script. Let's add it
to the git ignore list so the tree is marked as clean when the tools dir
exists.

Signed-off-by: Daniel Wagner <[email protected]>

Author: igaw

.gitignore

@@ -11,6 +11,7 @@ cscope.*
 compile_commands.json
 
 .build
+.build-tools
 .cache
 .vscode/