cmd/dist: make cc rule match what cmd/go uses

We added -I$GOROOT/pkg/$GOOS_$GOARCH in cmd/go
(I think for use by cgo and swig, primarily) but didn't
update cmd/dist. I was testing some other code and
found that my changes built with cmd/go but failed
during the initial bootstrap. Make them match again.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7707044

src/cmd/dist/build.c

@@ -939,6 +939,8 @@ install(char *dir)
 			}
 			vadd(&compile, "-I");
 			vadd(&compile, workdir);
+			vadd(&compile, "-I");
+			vadd(&compile, bprintf(&b, "%s/pkg/%s_%s", goroot, goos, goarch));
 			vadd(&compile, "-D");
 			vadd(&compile, bprintf(&b, "GOOS_%s", goos));
 			vadd(&compile, "-D");