flake8: fix warnings for unused imports

This is supposed to expose all primitives together, but to do that we
need to actually "use" each variable in `__all__`, which we... didn't.
Sorry about that.

mesonbuild/interpreter/primitives/__init__.py

@@ -12,6 +12,8 @@ __all__ = [
     'MesonVersionStringHolder',
     'DependencyVariableString',
     'DependencyVariableStringHolder',
+    'OptionString',
+    'OptionStringHolder',
 ]
 
 from .array import ArrayHolder