Enable clang-tidy on header files

clang-tidy appears to search for the .clang-tidy configuration file in the current working directory before searching the parent directories of the source file. Previously a dummy .clang-tidy file was placed in the build directory with the intent of avoiding running clang-tidy on external projects like Google Test. Unfortunately, since the build directory was the current working directory at build time this meant that header files were not checked by clang-tidy. This change fixes the problem by putting the dummy .clang-tidy file in the _deps directory that CMake uses for external projects.

To ensure that headers in new directories such as kleidicv_thread are checked by clang-tidy, make the header filter regular expression match everything.