fix missing symbol with rt

[pickx]

cherry-pick of the relevant commit from #42

[1arie1]

tests are failing. I think I need to resolve this. marked for myself

[1arie1]

The missing symbol is defined in cvlr-log. It is the intention that cvlr-asserts is linked together with cvlr-log.

The forward declaration in cvlr-assert is so that it can be used in debugging/tests without including the full cvlr-log dependency.

@pickx where is the example that cvlr-assert is linked without cvlr-log? Most users are expected to depend on cvlr that automatically includes all other dependencies