mac_framework.h: fix build with DEBUG_VFS_LOCKS and !MAC

I have such a custom kernel configuration and its build failed with:
linking kernel.full
ld: error: undefined symbol: mac_vnode_assert_locked

referenced by mac_framework.h:556 (/usr/devel/git/apu2c4/sys/security/mac/mac_framework.h:556)

tmpfs_vnops.o:(mac_vnode_check_stat)

referenced by mac_framework.h:556 (/usr/devel/git/apu2c4/sys/security/mac/mac_framework.h:556)

vfs_default.o:(mac_vnode_check_stat)

referenced by mac_framework.h:556 (/usr/devel/git/apu2c4/sys/security/mac/mac_framework.h:556)

ufs_vnops.o:(mac_vnode_check_stat)