List argument types in dummyRec->CreateWindow() function declaration

Will be needed for C23 builds, since C23 treats () as (void),
not as an unspecified argument list.

Fixes warning from Solaris Studio C compiler with -fd flag:
"dummy.h", line 55: warning: old style function declaration

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

src/dummy.h

@@ -52,7 +52,7 @@ typedef struct dummyRec
     int cursorFG, cursorBG;
 
     dummy_colors colors[1024];
-    Bool        (*CreateWindow)() ;     /* wrapped CreateWindow */
+    Bool        (*CreateWindow)(WindowPtr) ;     /* wrapped CreateWindow */
     Bool prop;
     /* XRANDR support begin */
     int num_screens;