Fix conan install by specifiyng zlib version

[hrabalik]

• fixes #215
• 'fltk/1.3.8' requires 'zlib/1.2.11'
• 'libxml2/2.9.12' requires 'zlib/1.2.12'
• to resolve the conflict, specify 'zlib/1.2.12' in the conanfile

[codecov-commenter]

Codecov Report

Merging #216 (d5e73a7) into main (2b9c3c9) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #216   +/-   ##
=======================================
  Coverage   80.00%   80.00%           
=======================================
  Files           3        3           
  Lines          30       30           
=======================================
  Hits           24       24           
  Misses          6        6           

Flag	Coverage Δ
Linux	?
Windows	80.00% <ø> (ø)
macOS	?

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