bench_ecdh: fix call to secp256k1_context_create #388

1. 
   apoelstra commented at 10:17 am on April 27, 2016: contributor
2. bench_ecdh: fix call to secp256k1_context_create f36f9c64bb
3. 
   apoelstra commented at 10:18 am on April 27, 2016: contributor

   Confirmed that this is the only call to context_create in the codebase that does not use one of the new constants.
4. FelixWeis cross-referenced this on May 6, 2016 from issue bench_ecdh not working by FelixWeis
5. 
   FelixWeis commented at 0:29 am on May 7, 2016: none

   tested, works
6. 
   peterdettman commented at 9:11 am on May 7, 2016: contributor

   Oops, I had noted this as long ago as #363 and forgot to send a separate PR.
7. 
   apoelstra commented at 3:35 pm on May 8, 2016: contributor

   @sipa @gmaxwell This is a one-line patch – can I get a quick ACK?
8. 
   sipa commented at 5:47 pm on May 8, 2016: contributor

   Quick ACK
9. sipa merged this on May 8, 2016
10. sipa closed this on May 8, 2016

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11. sipa referenced this in commit d2ee340561 on May 8, 2016
12. apoelstra deleted the branch on Jun 19, 2017

Contributors
apoelstra FelixWeis peterdettman sipa