Fix gnxi tests by regenerating code.

[robshakir]

 * (M) gnmi/model.go
  - Fix missing ytypes import.
 * (A) gnmi/modeldata/gostruct/gen.go
  - Add file such that "go generate" can be used to regenerate the
    model data package.
 * (M) gnmi/modeldata/gostruct/generated.go
  - Update generated code.

[codecov-io]

Codecov Report

Merging #19 into master will decrease coverage by 2.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage    17.3%   15.03%   -2.28%     
==========================================
  Files          12       12              
  Lines        2092     2408     +316     
==========================================
  Hits          362      362              
- Misses       1670     1986     +316     
  Partials       60       60

Impacted Files	Coverage Δ
gnmi/modeldata/gostruct/generated.go	0.12% <ø> (-0.04%)	⬇️
gnmi/model.go	19.23% <ø> (ø)	⬆️

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