[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.

Author: alfredfo

libc/src/sys/wait/wait4Impl.h

@@ -14,7 +14,7 @@
 #include "src/__support/error_or.h"
 #include "src/errno/libc_errno.h"
 
-#include <sys/signal.h>
+#include <signal.h>
 #include <sys/syscall.h> // For syscall numbers.
 #include <sys/wait.h>