Fix cmake script for bullet

[Tobias-Fischer]

In conda-forge we got this error:

CMake Error at bullet/CMakeLists.txt:37 (target_compile_options):
  Cannot specify compile options for target "PUBLIC" which is not built by
  this project.

This fixes it.

[chapulina]

Oops, thanks for the fix. Can you sign your commit for DCO? Thanks!

[codecov]

Codecov Report

Merging #297 (23aa79a) into ign-physics5 (f0257d6) will not change coverage.
The diff coverage is n/a.

❗ Current head 23aa79a differs from pull request most recent head cf7c1c9. Consider uploading reports for the commit cf7c1c9 to get more accurate results

@@              Coverage Diff              @@
##           ign-physics5     #297   +/-   ##
=============================================
  Coverage         75.54%   75.54%           
=============================================
  Files               127      127           
  Lines              5655     5655           
=============================================
  Hits               4272     4272           
  Misses             1383     1383           

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[Tobias-Fischer]

Done @chapulina