patch 8.1.0312: wrong type for flags used in signal handlers

Problem:    Wrong type for flags used in signal handlers.
Solution:   Use sig_atomic_t. (Dominique Pelle, closes #3356)

src/os_win32.h

@@ -89,6 +89,7 @@
 
 #define FNAME_ILLEGAL "\"*?><|" /* illegal characters in a file name */
 
+#include <signal.h>
 #include <stdlib.h>
 #include <time.h>
 #include <sys/types.h>