patch 9.0.1544: recent glibc marks sigset() as a deprecated

Problem:    Recent glibc marks sigset() as a deprecated.
Solution:   Use sigaction() in mch_signal() if possible. (Ozaki Kiichi,
            closes #12373)

src/os_win32.h

@@ -95,6 +95,8 @@
 # endif
 #endif
 
+typedef void (*sighandler_T)(int, int);
+
 /*
  * Win32 has plenty of memory, use large buffers
  */