Add missing sync.NewCond wrapper

[ncw]

I noticed recently while integrating go-deadlock that it was missing a wrapper for sync.NewCond.

Here is a little patch to add it.

Thank you for a great package :-)

---

This change is 

[ncw]

PS as far as I know sync.NewCond was in go1.0 so doesn't need any build tags.

[codecov]

Codecov Report

Merging #22 (8df648e) into master (4a6d8f3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   87.09%   87.09%           
=======================================
  Files           2        2           
  Lines         217      217           
=======================================
  Hits          189      189           
  Misses         20       20           
  Partials        8        8           

Impacted Files	Coverage Δ
deadlock.go	84.81% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4a6d8f3...8df648e. Read the comment docs.

[sasha-s]

Thank you for adding this.

[sasha-s]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ncw)

[ncw]

Thanks for this great package!

I included it in the talk I gave on deadlocks at gophercon UK :-)

The video isn't up yet, alas, but should appear here in due course: https://www.youtube.com/c/GopherConUK/videos

[sasha-s]

Thanks, will check it out when the video is up.
I looked code for your talk, it is great you are talking about recursive read locks (not obvious and bit me in the past).