preprocessor: Revert premature change

This part of the cleanup patch turns out to require more pieces to
function correctly.  I must have got confused over which tree I was
testing.  The very first map has a different pointer to the file name,
so doesn't match with a pointer compare. We were relying on that.

	* c-common.c (try_to_locate_new_include_insertion_point): Revert change.