Fix build after r314656

Some of the changes I introduced to use .ALLSRC were correct in spirit,
but incorrect in reality -- in particular, ../Makefile.inc hadn't been
pulled in via bsd.init.mk (via bsd.lib.mk, bsd.prog.mk), so the value
of .ALLSRC (evaluated immediately) was empty. .include bsd.init.mk
explicitly so we can be certain that the values used as dependencies in
the targets are defined when the target recipe has been evaluated.

Reminder: thou shalt separate out separate functional changes before
          committing them.

(YUGE) Pointyhat to:	ngie
In collaboration with:	bdrewery
MFC after:	1 month
Reported by:	Jenkins, cy, ler, O. Hartmann, Michael Butler
Sponsored by:	Dell EMC Isilon

usr.sbin/amd/amd/Makefile

@@ -6,7 +6,7 @@
 # $FreeBSD$
 #
 
-.include <src.opts.mk>
+.include <bsd.init.mk>
 
 .PATH:	${SRCTOP}/contrib/amd/amd
 

usr.sbin/amd/libamu/Makefile

@@ -5,6 +5,8 @@
 #
 # $FreeBSD$
 
+.include <bsd.init.mk>
+
 .PATH:	${SRCTOP}/contrib/amd/libamu \
 	${SRCTOP}/contrib/amd/conf/transp \
 	${SRCTOP}/contrib/amd/conf/mtab \