fix gridsample x86 warnings

[nihui]

No description provided.

[codecov-commenter]

Codecov Report

Merging #5096 (bf0b7e6) into master (54ab805) will increase coverage by 0.00%.
Report is 3 commits behind head on master.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5096   +/-   ##
=======================================
  Coverage   94.69%   94.70%           
=======================================
  Files         776      776           
  Lines      240163   240118   -45     
=======================================
- Hits       227429   227394   -35     
+ Misses      12734    12724   -10     

Files	Coverage Δ
...ayer/x86/gridsample_bilinear_apply_interpolation.h	98.29% <100.00%> (ø)
src/layer/x86/gridsample_compute_blob.h	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes