runtime/pprof: skip TestMapping if CGO is not available

The test requires cgo

Change-Id: I1bffee5f187afcf4b7e27516451c56ddfc263a26
Reviewed-on: https://go-review.googlesource.com/118638Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/pprof/proto_test.go

@@ -238,6 +238,7 @@ func TestProcSelfMaps(t *testing.T) {
 // that the runtime can't symbolize. See ./testdata/mappingtest.
 func TestMapping(t *testing.T) {
 	testenv.MustHaveGoRun(t)
+	testenv.MustHaveCGO(t)
 
 	prog := "./testdata/mappingtest"