simplify substitute_path_rule_matches using filename_ncmp

At the time this function was written, there was no filename_ncmp,
only FILENAME_CMP. So, in order to do an n-cmp, we had to make a local
copy of the first n characters of our string and use that to perform
the comparison. This patch simplifies the function's implementation,
now that we have filename_ncmp.

gdb/ChangeLog:

        * source.c (substitute_path_rule_matches): Simplify using
        filename_ncmp instead of FILENAME_CMP.

Tested on x86_64-linux.