Always declare "kill" in include/sys/signal.h

Always declare "kill" in include/sys/signal.h

Hi,

As I mentioned recently [1], newlib is providing a "kill" symbol to link
against, without declaring "kill" in signal.h. This is confusing for the
libgfortran build, which tries to link against kill (which succeeds), then
tries to use it (which triggers -Werror=implicit-function-declaration).

This patch implements my suggestion in that thread - making the declaration
of 'kill' in libc/include/sys/signal.h unconditional.

I've tested this by building a modified libgfortran on AArch64/ARM to see
that the Werror goes away, and the libgfortran build succeeds.

Is something like this OK for newlib? If so, can someone please commit
it on my behalf, as I have no commit access here.

Otherwise, what is your preferred direction for me to take this patch?

Thanks,
James