Move definition of SECURITY_WIN32 from makefiles into source.

This makes it easier for people to recompile the source in other
contexts or other makefiles.

windows/wingss.c

@@ -2,6 +2,7 @@
 
 #include "putty.h"
 
+#define SECURITY_WIN32
 #include <security.h>
 
 #include "pgssapi.h"

windows/winmisc.c

@@ -5,6 +5,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include "putty.h"
+#define SECURITY_WIN32
 #include <security.h>
 
 OSVERSIONINFO osVersion;