cross-build: Avoid adding <sys/cdefs.h> to SYSINCS

Summary:
The sys/cdefs.h in src is incompatible with glibc's sys/cdefs.h so
cross-building broke when the former was added to SYSINCS in 1c9ff80. This
commit adds a guard around that to only do that when building on FreeBSD. This
should fix github CI.

Test Plan:
Ran buildkernel using tools/build/make.py on linux in github CI and
locally on FreeBSD to double check nothing broke.

Reviewed by:    dim
MFC after:      3 days
Differential Revision: https://reviews.freebsd.org/D54385

Author: ayrtonm

tools/build/Makefile

@@ -351,10 +351,12 @@ 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"
 # Needed to build most of the things below, which include sys/cdefs.h either
 # directly or transitively
 SYSINCS+=	${SRCTOP}/sys/sys/_visible.h
 SYSINCS+=	${SRCTOP}/sys/sys/cdefs.h
+.endif
 
 # Needed to build config (since it uses libnv)
 SYSINCS+=	${SRCTOP}/sys/sys/_nv.h