Fixup include of gmp/config.mk to use new location
This wasn't spotted rightaway in
because the include-site deliberately ignored include-errors as
a Hack with the justification below:
Hack. The file gmp/config.mk doesn't exist yet after running ./configure in
the toplevel (ghc) directory. To let some toplevel make commands such as
sdist go through, right after ./configure, don't consider this an error.
This may have contributed to Trac #14891.