Fix -Wimplicit-function-declaration
Clang 16 will make -Wimplicit-function-declaration error by default.
For more information, see LWN.net [0] or LLVM's Discourse [1], gentoo-dev@ [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://archives.gentoo.org/gentoo-dev/message/
dd9f2d3082b8b6f8dfbccb0639e6e240
[3] hosted at lists.linux.dev.
Bug: https://bugs.gentoo.org/875527
Signed-off-by: Sam James <sam@gentoo.org>