Follow-up r308602: Don't add missing headers to .depend.tables.h.

This also avoids an error from egrep when a header is missing.  This can happen
with something like WITHOUT_BLUETOOTH set when searching for
$include_dir/netgraph/bluetooth/include/ng_btsocket.h.  The warning was
not an error (from set -e) due to being on the left side of a pipe.  Now the
all_headers list is only filled with existing headers.

Reviewed by:	ngie
MFC after:	3 days
Sponsored by:	Dell EMC Isilon

lib/libsysdecode/mktables

@@ -65,6 +65,7 @@ gen_table()
 	else
 		filter="egrep -v"
 	fi
+	[ -e "${include_dir}/${file}" ] || return 0
 	all_headers="${all_headers:+${all_headers} }${file}"
 	cat <<_EOF_
 TABLE_START(${name})