OpenBSD needs execinfo as well.

[3405691582]

Add OpenBSD to the CMake clause that adds libexecinfo.

Motivation:

swift test fails on some projects with ld: error: undefined reference due to --no-allow-shlib-undefined: backtrace; backtrace is provided by libexecinfo on OpenBSD.

Modifications:

Add OpenBSD to the CMake clause that adds libexecinfo.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

Thanks!