Fix HAVE_DECL_RES_INIT conditionals.

HAVE_DECL_RES_INIT is generated using AC_CHECK_DECLS. tinc checks this
symbol using #ifdef, which is wrong because (according to autoconf docs)
the symbol is always defined, it's just set to zero if the check failed.

This broke the Windows build starting from
0b310bf406dbe58afe37fa31156b9ea47599d7be, because it introduced this
conditional in code that's not excluded from the Windows build.