native: fix NEP17.Transfer cost

[fyrchik]

It was increased with the introducion of postTransfer.

[codecov]

Codecov Report

Merging #1617 (5310c7f) into master (f009e53) will increase coverage by 7.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1617      +/-   ##
==========================================
+ Coverage   75.43%   82.53%   +7.09%     
==========================================
  Files         241      241              
  Lines       19022    19022              
==========================================
+ Hits        14349    15699    +1350     
+ Misses       3845     2339    -1506     
- Partials      828      984     +156     

Impacted Files	Coverage Δ
pkg/core/native/native_nep17.go	74.53% <100.00%> (+1.24%)	⬆️
pkg/network/metrics/metrics.go	63.63% <0.00%> (-18.19%)	⬇️
pkg/compiler/codegen.go	90.55% <0.00%> (+0.39%)	⬆️
pkg/vm/stackitem/item.go	90.36% <0.00%> (+0.48%)	⬆️
pkg/rpc/server/server.go	77.45% <0.00%> (+0.55%)	⬆️
pkg/compiler/debug.go	92.79% <0.00%> (+0.90%)	⬆️
pkg/rpc/client/rpc.go	87.18% <0.00%> (+0.93%)	⬆️
pkg/vm/vm.go	93.88% <0.00%> (+1.06%)	⬆️
pkg/core/blockchain.go	76.71% <0.00%> (+1.10%)	⬆️
pkg/core/native/native_neo.go	70.32% <0.00%> (+1.15%)	⬆️
... and 30 more

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