libmach: fix build

I guess it would be too much to ask for gcc on my machine to give
the same errors as gcc on the builder machines.

R=ken2
CC=golang-dev
https://golang.org/cl/7686044

src/libmach/executable.c

@@ -1116,6 +1116,10 @@ machdotout(int fd, Fhdr *fp, ExecHdr *hp)
 	textsize = 0;
 	datasize = 0;
 	bsssize = 0;
+	symoff = 0;
+	symsize = 0;
+	pclnoff = 0;
+	pclnsize = 0;
 	for (i = 0; i < mp->ncmds; i++) {
 		MachCmd *c;