runtime/pprof, runtime/trace: stub some additional functions

These are necessary to import some parts of the net/http package.

src/runtime/trace/trace.go

@@ -0,0 +1,13 @@
+// Stubs for the runtime/trace package
+package trace
+
+import (
+	"errors"
+	"io"
+)
+
+func Start(w io.Writer) error {
+	return errors.New("not implemented")
+}
+
+func Stop() {}