lkdtm: split build into multiple source files

Kbuild lacks a way to do in-place objcopy or other modifications of
built targets, so in order to move functions into non-text sections
without renaming the kernel module, the build of lkdtm must be split
into separate source files. This renames lkdtm.c to lkdtm_core.c in
preparation for adding the source file for the .rodata section.
Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/misc/Makefile

@@ -57,3 +57,5 @@ obj-$(CONFIG_ECHO)		+= echo/
 obj-$(CONFIG_VEXPRESS_SYSCFG)	+= vexpress-syscfg.o
 obj-$(CONFIG_CXL_BASE)		+= cxl/
 obj-$(CONFIG_PANEL)             += panel.o
+
+lkdtm-$(CONFIG_LKDTM)		+= lkdtm_core.o