Ensure that a fake compilation database is generated for headers on targets.

Header files specified on a target won't be added to compile_commands.json
but we might want to run clang-tidy on them. Add them to
${TARGET}_compile_commands/compile_commands.json and run clang-tidy with that.
Merge pull request #3 from smspillaz/clang-tidy-target-cmake.item_3

Ensure that a fake compilation database is generated for headers on targets.