xvba_image.c: add extern keyword

The extern keyword is needed for instructing the linker to resolve
hw_image_hooks_glx, which is declared in another object. Failing to
do so results in an assert later on when using subpictures.

src/xvba_image.c

@@ -423,7 +423,7 @@ destroy_image(
 }
 
 #if USE_GLX
-const HWImageHooks hw_image_hooks_glx attribute_hidden;
+extern const HWImageHooks hw_image_hooks_glx attribute_hidden;
 #endif
 
 // Commit image to the HW