Fix a warning about an unused variable

*xi is only used if USE_IMLIB is defined, move the declaration inside
the #ifdef block.

w3mimg/x11/x11_w3mimg.c

@@ -698,8 +698,8 @@ static int
 x11_get_image_size(w3mimg_op * self, W3MImage * img, char *fname, int *w,
 		   int *h)
 {
-    struct x11_info *xi;
 #if defined(USE_IMLIB)
+    struct x11_info *xi;
     ImlibImage *im;
 #elif defined(USE_IMLIB2)
     Imlib_Image im;