Fix include paths with spaces in direct stanc calls

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Fixes #820

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.07%. Comparing base (6508f55) to head (877f7e2).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1226      +/-   ##
==========================================
- Coverage   91.88%   91.07%   -0.81%     
==========================================
  Files          15       15              
  Lines        6244     6252       +8     
==========================================
- Hits         5737     5694      -43     
- Misses        507      558      +51     

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