Unbreak the build for some modules that depend on -std flags to be passed

through via CFLAGS, as noted by Jenkins when building sparc64 by re-adding
bsd.sys.mk to bsd.kmod.mk:
https://jenkins.freebsd.org/job/FreeBSD_HEAD_sparc64/225/changes

Tested with:
    % cd /sys/modules/bce; make obj; make depend; make all

share/mk/bsd.kmod.mk

@@ -13,3 +13,5 @@ SYSDIR=	${_dir}
 .endif
 
 .include "${SYSDIR}/conf/kmod.mk"
+
+.include <bsd.sys.mk>