Fixed compile warnings for gcc linux build. [-Wunused-parameter] [-Wunused-but-set-variable]

[proydakov]

Hi, NCNN Team.

I fixed 2 small compile warnings for gcc linux build:

https://github.com/Tencent/ncnn/runs/1239480361?check_suite_focus=true

Could you verify and accept my PR, pls?

/home/runner/work/ncnn/ncnn/src/c_api.cpp:161:54: warning: unused parameter ‘opt’ [-Wunused-parameter]
int ncnn_option_get_use_vulkan_compute(ncnn_option_t opt)
^~~

/home/runner/work/ncnn/ncnn/src/layer/interp.cpp: In member function ‘virtual int ncnn::Interp::forward(const ncnn::Mat&, ncnn::Mat&, const ncnn::Option&) const’:
/home/runner/work/ncnn/ncnn/src/layer/interp.cpp:411:9: warning: variable ‘channels’ set but not used [-Wunused-but-set-variable]
int channels = bottom_blob.c;
^~~~~~~~

[codecov-io]

Codecov Report

Merging #2198 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2198   +/-   ##
=======================================
  Coverage   86.47%   86.47%           
=======================================
  Files         511      511           
  Lines       83225    83223    -2     
=======================================
- Hits        71972    71971    -1     
+ Misses      11253    11252    -1     

Impacted Files	Coverage Δ
src/layer/interp.cpp	96.62% <ø> (+0.28%)	⬆️

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[nihui]

Thanks!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.47%. Comparing base (9be3f07) to head (1cc93ed).
⚠️ Report is 1880 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2198   +/-   ##
=======================================
  Coverage   86.47%   86.47%           
=======================================
  Files         511      511           
  Lines       83225    83223    -2     
=======================================
- Hits        71972    71971    -1     
+ Misses      11253    11252    -1     

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