aws-lc-sys: make bindgen optional

[justsmth]

Description of changes:

Make bindgen always an optional dependency

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.51%. Comparing base (c358484) to head (62641e9).
⚠️ Report is 287 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #949      +/-   ##
==========================================
- Coverage   95.80%   92.51%   -3.29%     
==========================================
  Files          61       74      +13     
  Lines        8143     9850    +1707     
  Branches        0     9850    +9850     
==========================================
+ Hits         7801     9113    +1312     
- Misses        342      450     +108     
- Partials        0      287     +287     

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