[Libomptarget] Add header files as a dependency to CMake target

This patch manually adds the runtime include files to the list of
dependencies when we build the bitcode runtime library. Previously if
only the header was changed we would not recompile the source files.
The solution used here isn't optimal because every source file not has a
dependency on each header file regardless of if it was actually used by
that file.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D119254