test: use %zd for size_t in mbr_test_run()

For printing size_t we must use %zd and not %ld to avoid
a -Wformat error on 32-bit systems.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/test/cmd/mbr.c b/test/cmd/mbr.c
index 16a8bc3..d137378 100644
--- a/test/cmd/mbr.c
+++ b/test/cmd/mbr.c
@@ -257,8 +257,8 @@
 		strlen(mbr_parts_p3) +
 		max(strlen(mbr_parts_p4), strlen(mbr_parts_p5)) +
 		strlen(mbr_parts_tail);
-	ut_assertf(sizeof(mbr_parts_buf) >= mbr_parts_max, "Buffer avail: %ld; buffer req: %ld\n",
-		sizeof(mbr_parts_buf), mbr_parts_max);
+	ut_assertf(sizeof(mbr_parts_buf) >= mbr_parts_max, "Buffer avail: %zd; buffer req: %ld\n",
+		   sizeof(mbr_parts_buf), mbr_parts_max);
 
 	mbr_wbuf = map_sysmem(mbr_wa, BLKSZ);
 	ebr_wbuf = map_sysmem(ebr_wa, BLKSZ);