go.mod: update github.com/google/pprof

We want the newer version pprof to support merging column field in Line.

Change-Id: I5c8122f67a38c67b21d1ec8ef7441c5c2fce6e57
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/621475
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>