Change import paths used in crypto/secp256k1/dummy.go

[cce]

Summary

Since the merge of #2852 go mod tidy and go mod vendor see paths in github.com/ethereum/go-ethereum/crypto/secp256k1/libsecp256k1 as a dependency, and go mod tidy adds it to go.mod.

Test Plan

All tests should pass as before.

[codecov-commenter]

Codecov Report

Merging #2881 (aab9db1) into master (08d31fa) will decrease coverage by 3.77%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2881      +/-   ##
==========================================
- Coverage   47.00%   43.23%   -3.78%     
==========================================
  Files         355      355              
  Lines       57195    57195              
==========================================
- Hits        26884    24726    -2158     
- Misses      27239    29479    +2240     
+ Partials     3072     2990      -82     

Impacted Files	Coverage Δ
crypto/merklearray/worker.go	0.00% <0.00%> (-100.00%)	⬇️
crypto/compactcert/structs.go	0.00% <0.00%> (-100.00%)	⬇️
agreement/agreementtest/keyManager.go	0.00% <0.00%> (-100.00%)	⬇️
daemon/algod/api/server/lib/middlewares/auth.go	0.00% <0.00%> (-100.00%)	⬇️
agreement/agreementtest/simulate.go	0.00% <0.00%> (-87.68%)	⬇️
cmd/algofix/deadlock.go	3.12% <0.00%> (-84.38%)	⬇️
crypto/merklearray/partial.go	0.00% <0.00%> (-84.10%)	⬇️
ledger/onlineacct.go	17.64% <0.00%> (-82.36%)	⬇️
crypto/onetimesig.go	0.00% <0.00%> (-76.28%)	⬇️
data/account/rootInstall.go	0.00% <0.00%> (-75.00%)	⬇️
... and 93 more

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