xwin: drop redundant declaration of winValidateArgs()

../hw/xwin/InitOutput.c:89:2: warning: redundant redeclaration of ‘winValidateArgs’ [-Wredundant-decls]
   89 |  winValidateArgs(void);
      |  ^~~~~~~~~~~~~~~

In file included from ../hw/xwin/InitOutput.c:35:
../hw/xwin/win.h:1008:1: note: previous declaration of ‘winValidateArgs’ was here

 1008 | winValidateArgs(void);
      | ^~~~~~~~~~~~~~~

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

hw/xwin/InitOutput.c

@@ -91,9 +91,6 @@ void
 void
  winLogVersionInfo(void);
 
-Bool
- winValidateArgs(void);
-
 #ifdef RELOCATE_PROJECTROOT
 const char *winGetBaseDir(void);
 #endif