Move style-check to separate script

This avoids having to copy out of .gitlab-ci.yml before submitting an MR
to do a style check.

Could become a meson target at some point.

.gitlab-ci.yml

@@ -74,8 +74,7 @@ test:
     - apt-get -y build-dep .
     - apt-get -y install clang-tidy
     - ninja -C _build test
-    - cd _build
-    - clang-tidy --checks=-clang-diagnostic-missing-braces,readability-braces-around-statements, --warnings-as-errors=readability-braces-around-statements -extra-arg=-Wno-unknown-warning-option ../src/*.c ../eek/*.c ../eekboard/*.c
+    - tools/style-check _build
 
 check_release:
   stage: test