tools/build: stage cdefs.h on MidnightBSD hosts

Include sys/sys/cdefs.h in the legacy header set when bootstrapping on MidnightBSD so host tools that pull in sys/font.h also see __nonstring.

AI-Assisted-by: OpenAI Codex (GPT-5)
Signed-off-by: Lucas Holt <luke@foolishgames.com>

Author: laffer1

tools/build/Makefile

@@ -231,7 +231,7 @@ MSDOSFSINCS+=	${SRCTOP}/sys/fs/msdosfs/fat.h
 MSDOSFSINCS+=	${SRCTOP}/sys/fs/msdosfs/msdosfsmount.h
 DISKINCS+=	${SRCTOP}/sys/sys/disk/bsd.h
 
-.if ${.MAKE.OS} == "FreeBSD"
+.if ${.MAKE.OS} == "FreeBSD" || ${.MAKE.OS} == "MidnightBSD"
 # Needed to build most of the things below, which include sys/cdefs.h either
 # directly or transitively
 SYSINCS+=	${SRCTOP}/sys/sys/cdefs.h