While for in-kernel we already have LINUXKPI_INCLUDES in kern.pre.mk
for kmod builds we've not had a common define to use leading to various
spellings of include paths.
In order for the include list to be expanded more easily in the future,
e.g., adding the "dummy" includes (for all) and to harmonize code,
duplicate LINUXKPI_INCLUDES to kmod.mk and use it for all module Makefiles.
MFC after: 1 week
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D36683
514fb38721