Fixup include of gmp/config.mk to use new location

This wasn't spotted rightaway in
8f0b2f5e
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 #14891.

libraries/integer-gmp/gmp/ghc.mk

@@ -56,7 +56,7 @@ ifneq "$(CLEANING)" "YES"
 # 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.
--include libraries/integer-gmp/gmp/config.mk
+-include libraries/integer-gmp/dist-install/build/gmp/config.mk
 endif
 
 gmp_CC_OPTS += $(addprefix -I,$(GMP_INCLUDE_DIRS))