src: fix build with older glibc

src: fix build with older glibc

glibc before 2.19 missed the definition of IPPROTO_MH. This leads to
build failure:

parser_bison.y: In function 'nft_parse':
parser_bison.y:3793:21: error: 'IPPROTO_MH' undeclared (first use in this function)
    | MH { $$ = IPPROTO_MH; }
                     ^

Since we have a local definition of IPPROTO_MH in headers.h use that to
fix the build.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>