tools.build: add sys/cdefs.h to SYSINCS, since lots of other headers use it

This is needed to let the legacy stage compile against newer versions of
sys/font.h, which transitively includes sys/cdefs.h, and requires the
new __nonstring macro from it.

Fixes:		e2c93ed09f
MFC after:	3 days

tools/build/Makefile

@@ -351,6 +351,10 @@ MSDOSFSINCS+=	${SRCTOP}/sys/fs/msdosfs/fat.h
 MSDOSFSINCS+=	${SRCTOP}/sys/fs/msdosfs/msdosfsmount.h
 DISKINCS+=	${SRCTOP}/sys/sys/disk/bsd.h
 
+# Needed to build most of the things below, which include sys/cdefs.h either
+# directly or transitively
+SYSINCS+=	${SRCTOP}/sys/sys/cdefs.h
+
 # Needed to build config (since it uses libnv)
 SYSINCS+=	${SRCTOP}/sys/sys/_nv.h
 SYSINCS+=	${SRCTOP}/sys/sys/nv.h ${SRCTOP}/sys/sys/cnv.h \