syscall: fix mksysnum_dragonfly.pl

The format of the DragonFly BSD syscalls.master file has changed
slightly - update mksysnum_dragonfly.pl to match.

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/71460044

src/pkg/syscall/mksysnum_dragonfly.pl

@@ -20,7 +20,7 @@ const (
 EOF
 
 while(<>){
-	if(/^([0-9]+)\s+STD\s+\S+\s+({ \S+\s+(\w+).*)$/){
+	if(/^([0-9]+)\s+STD\s+({ \S+\s+(\w+).*)$/){
 		my $num = $1;
 		my $proto = $2;
 		my $name = "SYS_$3";