lint: Check for missing braces

The `eek/layersurface.c` file should be excluded because it's an imported, "foreign" source, but clang-tidy doesn't seem to have an annotation for that.

An alternative would have been to exclude it in Meson and do the check there, but that requires clang-tidy, raising the barrier to contribute of Squeekboard even more (it already requires libfeedback, which isn't packaged widely).

.gitlab-ci.yml

@@ -136,7 +136,10 @@ test:
     - build_meson
   script:
     - 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
 
 check_release:
   <<: *tags