acx_nlnetlabs.m4: fix -Wstrict-prototypes

acx_nlnetlabs.m4: fix -Wstrict-prototypes

Fix -Wstrict-prototypes as it's easy to do and avoids future warnings. Noticed
when doing Clang 16 / GCC 14 / C23 work.

For more information, see LWN.net [0] or LLVM's Discourse [1], the Gentoo wiki [2],
or the (new) c-std-porting mailing list [3].

[0] https://lwn.net/Articles/913505/
[1] https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213
[2] https://wiki.gentoo.org/wiki/Modern_C_porting
[3] hosted at lists.linux.dev.

Signed-off-by: Sam James <sam@gentoo.org>