After rG1111a44301da, MINIMAL kernel stopped building.
```
linking kernel.full
ld: error: undefined symbol: sysctl___vfs_ffs
>>> referenced by ffs_subr.c
>>> ffs_subr.o:(sysctl___vfs_ffs_prttimechgs)
*** [kernel.full] Error code 1
make[2]: stopped making "all" in /usr/obj/usr/src/amd64.amd64/sys/MINIMAL
make[2]: 1 error
make[2]: stopped making "all" in /usr/obj/usr/src/amd64.amd64/sys/MINIMAL
143.83 real 2018.96 user 165.62 sys
make[1]: stopped making "buildkernel" in /usr/src
make: stopped making "buildkernel" in /usr/src
```
This happens because `sys/conf/files` has this:
```
ufs/ffs/ffs_subr.c optional ffs | geom_label
```
This patch fixes the problem by moving it to a separate file.