libdiskconfig: fix build (bad type in offset calculation)

Change-Id: I1acc490943f961a039acd3d40db825069b725abb
Signed-off-by: JP Abgrall <jpa@google.com>
diff --git a/libdiskconfig/config_mbr.c b/libdiskconfig/config_mbr.c
index 82d0066..7b6ca1c 100644
--- a/libdiskconfig/config_mbr.c
+++ b/libdiskconfig/config_mbr.c
@@ -221,7 +221,7 @@
         /* DO NOT DEREFERENCE */
         struct pc_boot_record *mbr = (void *)PC_MBR_DISK_OFFSET;
         /* grab the offset in mbr where to write mbr signature. */
-        item->offset = (loff_t)((uint32_t)((uint8_t *)(&mbr->mbr_sig)));
+        item->offset = (loff_t)((uintptr_t)((uint8_t *)(&mbr->mbr_sig)));
     }
 
     *((uint16_t*)item->data) = PC_BIOS_BOOT_SIG;