extmod/vfs: Expose mp_vfs_mount_t type.

It should only be used for low-level things and with caution, for example
putting mounted VFS data in ROM or the static data section.
diff --git a/py/mpstate.h b/py/mpstate.h
index 9c73f77..daf085a 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -166,8 +166,8 @@
     #endif
 
     #if MICROPY_VFS
-    struct _vfs_mount_t *vfs_cur;
-    struct _vfs_mount_t *vfs_mount_table;
+    struct _mp_vfs_mount_t *vfs_cur;
+    struct _mp_vfs_mount_t *vfs_mount_table;
     #endif
 
     //