hw/xwin: Fix const warning in winCheckDisplayNumber()

InitOutput.c:1032:19: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

hw/xwin/InitOutput.c

@@ -1010,7 +1010,7 @@ winCheckDisplayNumber(void)
     int nDisp;
     HANDLE mutex;
     char name[MAX_PATH];
-    char *pszPrefix = '\0';
+    const char *pszPrefix = '\0';
     OSVERSIONINFO osvi = { 0 };
 
     /* Check display range */