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).

doc/hacking.md

@@ -113,7 +113,7 @@ User interface modules should:
 
 Code submitted should roughly match the style of surrounding code. Things that will *not* be accepted are ones that often lead to errors:
 
-- skipping brackets `{}` after every `if()`, `else`, and similar
+- skipping brackets `{}` after every `if()`, `else`, and similar ([SCI CERT C: EXP19-C](https://wiki.sei.cmu.edu/confluence/display/c/EXP19-C.+Use+braces+for+the+body+of+an+if%2C+for%2C+or+while+statement))
 
 Bad example: