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.