net: fix implicit kmod.h usage in bridge/br_stp_if.c

To fix this, once the implicit presence of module.h is removed:

net/bridge/br_stp_if.c: In function ‘br_stp_start’:
net/bridge/br_stp_if.c:131: error: implicit declaration of function ‘call_usermodehelper’
net/bridge/br_stp_if.c:131: error: ‘UMH_WAIT_PROC’ undeclared (first use in this function)

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