kernel: sys.c implicitly relies on kmod.h for usermodehelper

kernel: sys.c implicitly relies on kmod.h for usermodehelper

In preparation to clean up the pervasive module.h usage which
in turn brings in every header possible, testing reveals that
this file gets the usermodehelper functions via that path.

Explicitly include kmod.h so that we don't things like:

kernel/sys.c:317: error: implicit declaration of function ‘usermodehelper_disable’
kernel/sys.c:1816: error: implicit declaration of function ‘call_usermodehelper_setup’
kernel/sys.c:1822: error: implicit declaration of function ‘call_usermodehelper_setfns’
kernel/sys.c:1824: error: implicit declaration of function ‘call_usermodehelper_exec’

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>