[clang-tidy] Set HeaderFilterRegex to '.*'

This matches now all headers in the repository, instead of just the
ones in the emper/ subdirectory, for example the
/tests/fixtures.network.hpp header.

.clang-tidy

@@ -20,4 +20,4 @@ WarningsAsErrors: >
   readability-*,
   performance-*,
 
-HeaderFilterRegex: emper/*
+HeaderFilterRegex: .*