optimize scanning of vulkan shader files in ncnn_add_layer()

[maxint]

用 cmake version 3.24.2 在 Ubuntu-16.04 下 cmake configure 很卡，需要优化下

[tencent-adm]

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• cmake/ncnn_add_layer.cmake: Language not supported

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.52%. Comparing base (71ed023) to head (f28ce69).
Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6019       +/-   ##
===========================================
+ Coverage   94.43%   95.52%    +1.08%     
===========================================
  Files         468      825      +357     
  Lines      133738   269647   +135909     
===========================================
+ Hits       126289   257567   +131278     
- Misses       7449    12080     +4631     

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[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	16465128	16465128	0 😘
armhf	7335260	7335260	0 😘
aarch64	10704800	10704800	0 😘

[nihui]

Thanks for your contribution !