st/wgl: init a variable to silence MinGW warning

MinGW release build says 'value' may be used before being initialized.

Reviewed-by: Neha Bhende <bhenden@vmware.com>

src/gallium/state_trackers/wgl/stw_ext_pixelformat.c

@@ -487,7 +487,7 @@ wglGetPixelFormatAttribfvARB(HDC hdc, int iPixelFormat, int iLayerPlane,
    (void) hdc;
 
    for (i = 0; i < nAttributes; i++) {
-      int value;
+      int value = 0;
 
       if (!stw_query_attrib(iPixelFormat, iLayerPlane,
                              piAttributes[i], &value))