Avoid a compiler warning about a potentially unused variable.

There is no assignment to the "ret" variable if X_DRI2SwapBuffers is
not defined. In this case, the earlier explicit "return 0" is likely
to be used, but the compiler can't be sure of that, (nor can I for
that matter).

We cover this case by explicitly initializing "ret" to 0.

src/glx/dri2_glx.c

@@ -494,7 +494,7 @@ dri2SwapBuffers(__GLXDRIdrawable *pdraw, int64_t target_msc, int64_t divisor,
     struct dri2_screen *psc = (struct dri2_screen *) priv->base.psc;
     struct dri2_display *pdp =
 	(struct dri2_display *)dpyPriv->dri2Display;
-    CARD64 ret;
+    CARD64 ret = 0;
 
 #ifdef __DRI2_FLUSH
     if (psc->f)