bugfix: Load libxcb.so if the main library doesn't load

[notgull]

Closes #785

@ids1024 Could you verify that this branch works on NetBSD?

[codecov]

Codecov Report

Base: 13.70% // Head: 13.70% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (f5ae460) compared to base (c8b4c9b).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #787      +/-   ##
==========================================
- Coverage   13.70%   13.70%   -0.01%     
==========================================
  Files         142      142              
  Lines      120070   120083      +13     
==========================================
  Hits        16454    16454              
- Misses     103616   103629      +13     

Flag	Coverage Δ
tests	13.70% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
x11rb/src/xcb_ffi/raw_ffi/ffi.rs	0.00% <0.00%> (ø)

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[psychon]

I'm rejecting this as Eduardo seems to agree that this should be properly figured out first. But thanks for turning my rambling into a PR.