chore(deps): pin pprof

[nolanmar511]

@google-cloud/profiler depends heavily on the pprof module.

We want to be sure that changes to pprof that introduce bugs or problems in @google-cloud/profiler can be caught by @google-cloud/profiler's presubmit tests, rather than immediately impacting @google-cloud/profiler as its running in prod.

This will allow us to concentrate testing in this repository, rather than duplicate all testing here and in https://github.com/google/pprof-nodejs.

[codecov]

Codecov Report

Merging #539 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #539   +/-   ##
=======================================
  Coverage   75.73%   75.73%           
=======================================
  Files           7        7           
  Lines         272      272           
  Branches       56       56           
=======================================
  Hits          206      206           
  Misses         50       50           
  Partials       16       16

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