cmd/pprof: update vendored github.com/google/pprof

cmd/pprof: update vendored github.com/google/pprof

Pull in the latest published version of github.com/google/pprof
as part of the continuous process of keeping Go's dependencies
up to date. Done with:

go get github.com/google/pprof
go mod tidy
go mod vendor

For #36905.
Fixes #65741.

Change-Id: Ice7b085c03ff69be97929cbe47bfd91954907529
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/564636
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>