Force MK_INCLUDES for the legacy stage

As legacy executes "make installincludes" we don't want it to be
disabled by a src.conf setting.

Reviewed by:	imp
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D2143

Makefile.inc1

@@ -259,7 +259,8 @@ BMAKE=		MAKEOBJDIRPREFIX=${WORLDTMP} \
 		-DNO_PIC MK_PROFILE=no -DNO_SHARED \
 		-DNO_CPU_CFLAGS MK_WARNS=no MK_CTF=no \
 		MK_CLANG_EXTRAS=no MK_CLANG_FULL=no \
-		MK_LLDB=no MK_TESTS=no
+		MK_LLDB=no MK_TESTS=no \
+		MK_INCLUDES=yes 
 
 # build-tools stage
 TMAKE=		MAKEOBJDIRPREFIX=${OBJTREE} \