Fix duplicate osl lexing symbols when building as static libraries

[brechtvl]

Description

Like other oslcomp source files that are duplicated in oslexec, this duplication should only be done for shared library builds where the symbols are hidden from each other.

Tests

Not applicable.

Checklist:

• [x] I have read the contribution guidelines.
• [x] I have previously submitted a Contributor License Agreement.
• [x] I have updated the documentation, if applicable.
• [ ] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [x] My code follows the prevailing code style of this project.