hw/xwin: Fix building with -fno-common

Provide an actual definition of noDriExtension where used, rather than a
tentative definition in a header, to fix compilation with -fno-common
(the default with gcc 10).

hw/xwin/InitOutput.c

@@ -111,6 +111,8 @@ static PixmapFormatRec g_PixmapFormats[] = {
     {32, 32, BITMAP_SCANLINE_PAD}
 };
 
+static Bool noDriExtension;
+
 static const ExtensionModule xwinExtensions[] = {
 #ifdef GLXEXT
 #ifdef XWIN_WINDOWS_DRI

hw/xwin/dri/windowsdri.h

@@ -25,6 +25,5 @@
 #define windowsdri_h
 
 void WindowsDRIExtensionInit(void);
-Bool noDriExtension;
 
 #endif /* windowsdri_h */