[libc] [NFC] wait4Impl: include <signal.h> instead of <sys/signal.h>

sys/signal.h currently does not exist in LLVM-libc. Both glibc and
musl provides this header with simply:

> #include <signal.h>
(and a warning in musl libc)

If you try to build LLVM-libc in a sysroot with only LLVM-libc headers
and linux-headers installed then this will fail the build due to
missing headers. However, if --sysroot is not passed, then the
LLVM-libc build will pick up the header from the system which in turn
includes LLVM-libc's signal.h.

LLVM-libc includes <signal.h> in all other cases, so this simply
matches that. Providing <sys/signal.h> could also be done (instead)
for glibc compatibility.