ntdll: Include <signal.h> where needed in the Unix library.

dlls/ntdll/unix/process.c

@@ -28,6 +28,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <signal.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>