remove 'sys/sysctl.h', it was a duplicate and no sysctl functionality is used
in geom_ctl.c. The duplicate here is how I wound up looking at the rest of the
header files.
remove 'opt_geom.h', geom_ctl.c doesn't use any kernel options.
remove 'sys/bio.h', requirements for bio moved out in r112534.
remove 'sys/lock.h' and 'sys/mutex.h', looks like the last thing that required
these headers were DROP_GIANT and PICKUP_GIANT, which were removed in
r115624.
It also appears that geom_ctl.c doesn't require anything from 'sys/disk.h' and
'sys/kernel.h'.
unless I'm missing something glaringly obvious..