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.

Change-Id: I2a48e912712bc916c9d749acb1550682f919477e
Reviewed-on: https://go-review.googlesource.com/c/go/+/519657
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>