Add the text '@generated' to src.conf.5

This is a cross-tool approach to identifying generated code. Some tools,
notably phabricator, handle this marker specially.  See
https://reviews.freebsd.org/differential/diff/42870/ for such an
example.

share/man/man5/src.conf.5

@@ -1,6 +1,7 @@
 .\" DO NOT EDIT-- this file is generated by tools/build/options/makeman.
 .\" $FreeBSD$
-.Dd May 15, 2018
+.\" @generated
+.Dd May 23, 2018
 .Dt SRC.CONF 5
 .Os
 .Sh NAME

tools/build/options/makeman

@@ -141,9 +141,11 @@ main()
 	echo "building src.conf.5 man page from files in ${PWD}" >&2
 
 	fbsdid='$'FreeBSD'$'
+  generated='@'generated
 	cat <<EOF
 .\" DO NOT EDIT-- this file is generated by tools/build/options/makeman.
 .\" ${fbsdid}
+.\" ${generated}
 .Dd $(echo $(LC_TIME=C date +'%B %e, %Y'))
 .Dt SRC.CONF 5
 .Os