(partially) Fix Bug#55227 Fix compiler warnings in innodb with gcc 4.6

Fix compiler warning:
btr/btr0cur.c: In function 'btr_free_externally_stored_field':
btr/btr0cur.c:4281:16: error: variable 'rec_block' set but not used [-Werror=unused-but-set-variable]

storage/innodb_plugin/btr/btr0cur.c

@@ -4278,12 +4278,17 @@ btr_free_externally_stored_field(
 	}
 
 	for (;;) {
+#ifdef UNIV_SYNC_DEBUG
 		buf_block_t*	rec_block;
+#endif /* UNIV_SYNC_DEBUG */
 		buf_block_t*	ext_block;
 
 		mtr_start(&mtr);
 
-		rec_block = buf_page_get(page_get_space_id(
+#ifdef UNIV_SYNC_DEBUG
+		rec_block =
+#endif /* UNIV_SYNC_DEBUG */
+			buf_page_get(page_get_space_id(
 						 page_align(field_ref)),
 					 rec_zip_size,
 					 page_get_page_no(