Fix build error with NCNN_PIXEL_DRAWING off

[shatyuka]

Introduced in #5324

[tencent-adm]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.58%. Comparing base (4797d19) to head (61e5702).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #5346       +/-   ##
===========================================
- Coverage   94.48%   89.58%    -4.90%     
===========================================
  Files         778      262      -516     
  Lines      243352    74229   -169123     
===========================================
- Hits       229921    66495   -163426     
+ Misses      13431     7734     -5697     

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[nihui]

Thanks for your contribution !