Makefile.am: don't silence -Wimplicit-function-declaration

Makefile.am: don't silence -Wimplicit-function-declaration

This becomes an error in GCC 14 and Clang 16. It's a common
misconception that these warnings are invalid or simply noise for
Bison/parser files, but even if that were true, we'd need to handle it
somehow anyway. Silencing them does nothing, so stop doing that.

Further, I don't actually get any warnings to fix with bison-3.8.2. This
mirrors changes we've done in other netfilter.org projects.

Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Phil Sutter <phil@nwl.cc>