hw/xwin: fmemopen is available on cygwin but not MinGW

This is a follow-up to commit 0659437f5e.

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

hw/xwin/winprefs.c

@@ -645,6 +645,7 @@ winPrefsLoadPreferences(char *path)
 
     if (path)
         prefFile = fopen(path, "r");
+#ifdef __CYGWIN__
     else {
         char defaultPrefs[] =
             "MENU rmenu {\n"
@@ -656,6 +657,7 @@ winPrefsLoadPreferences(char *path)
         path = "built-in default";
         prefFile = fmemopen(defaultPrefs, strlen(defaultPrefs), "r");
     }
+#endif
 
     if (!prefFile) {
         ErrorF("LoadPreferences: %s not found\n", path);