utils: fix one compilation error with --with-mini-gmp

utils: fix one compilation error with --with-mini-gmp

Restore some code that is needed, until we have a version of gmp_printf
that takes variable arguments.

In file included from ../include/utils.h:12:0,
                 from ../include/nftables.h:6,
                 from ../include/rule.h:5,
                 from segtree.c:15:
segtree.c: In function ‘ei_insert’:
../include/gmputil.h:12:20: error: too many arguments to function ‘mpz_printf’
 #define gmp_printf mpz_printf

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>