fix cmake

[wkcn]

Hi there,
When building ncnn, it outputs the following message:

CMake Error at src/CMakeLists.txt:413 (if):
  if given arguments:

    "DEFINED" "ANDROID_NDK_MAJOR" "AND" "GREATER" "20"

  Unknown arguments specified


-- Configuring incomplete, errors occurred!

I try to fix it.

[codecov-commenter]

Codecov Report

Merging #1823 into master will decrease coverage by 4.41%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #1823      +/-   ##
==========================================
- Coverage   77.00%   72.58%   -4.42%     
==========================================
  Files         383      294      -89     
  Lines       53567    35708   -17859     
==========================================
- Hits        41251    25920   -15331     
+ Misses      12316     9788    -2528     

Impacted Files	Coverage Δ
src/layer/x86/convolutiondepthwise_x86.cpp	57.60% <25.00%> (ø)
src/mat.h	60.36% <0.00%> (-12.35%)	⬇️
src/layer/crop.cpp	40.44% <0.00%> (-10.51%)	⬇️
src/layer/arm/convolution_1x1.h	56.50% <0.00%> (-4.42%)	⬇️
src/layer/noop.cpp	87.50% <0.00%> (-4.17%)	⬇️
src/layer/arm/convolution_3x3.h	49.12% <0.00%> (-3.89%)	⬇️
src/layer/arm/convolution_1x1_pack4to1.h	92.96% <0.00%> (-3.07%)	⬇️
src/layer/arm/convolution_1x1_pack4.h	93.58% <0.00%> (-2.93%)	⬇️
src/layer/arm/convolution_1x1_pack4to1_bf16s.h	90.06% <0.00%> (-2.31%)	⬇️
src/layer/arm/requantize_arm.cpp	37.20% <0.00%> (-1.93%)	⬇️
... and 150 more

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[nihui]

赞美大佬！