syscall: remove unnecessary semicolon from mksyscall.pl

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/5495098

src/pkg/syscall/mksyscall.pl

@@ -232,7 +232,7 @@ while(<>) {
 		$text .= "\t}\n";
 	} elsif ($do_errno) {
 		$text .= "\tif e1 != 0 {\n";
-		$text .= "\t\terr = e1;\n";
+		$text .= "\t\terr = e1\n";
 		$text .= "\t}\n";
 	}
 	$text .= "\treturn\n";