Fix Windows build.

GlobalMemoryStatusEx actually requires _WIN32_WINNT to be defined as
0x0501 (Windows XP) for availability.

For completeness, I bumped WIN32_WINNT in Ticker and OSThreads as well.

Signed-off-by: Austin Seipp <austin@well-typed.com>

rts/win32/OSMem.c

@@ -6,6 +6,8 @@
  *
  * ---------------------------------------------------------------------------*/
 
+#define _WIN32_WINNT 0x0501
+
 #include "Rts.h"
 #include "sm/OSMem.h"
 #include "RtsUtils.h"

rts/win32/OSThreads.c

@@ -7,7 +7,7 @@
  *
  * --------------------------------------------------------------------------*/
 
-#define _WIN32_WINNT 0x0500
+#define _WIN32_WINNT 0x0501
 
 #include "Rts.h"
 #include <windows.h>

rts/win32/Ticker.c

 /*
  * RTS periodic timers.
- * 
+ *
  */
-#define _WIN32_WINNT 0x0500
+#define _WIN32_WINNT 0x0501
 
 #include "Rts.h"
 #include "Ticker.h"