fix invalid type for default_options

We have two checks for the type accepted here. One is the basic
typed_kwargs type-checking, which declares it accepts:

  str | include_directories

The other is the custom validator which further narrows it to strings
with certain option-like properties (needs to be an = assignment).

The former is obviously wrong, which doesn't really matter all that much
but still isn't very nice...

Introduced in commit f34013fb08.

mesonbuild/interpreter/type_checking.py

@@ -328,7 +328,7 @@ INCLUDE_DIRECTORIES: KwargInfo[T.List[T.Union[str, IncludeDirs]]] = KwargInfo(
 # for cases like default_options and override_options
 DEFAULT_OPTIONS: KwargInfo[T.List[str]] = KwargInfo(
     'default_options',
-    ContainerTypeInfo(list, (str, IncludeDirs)),
+    ContainerTypeInfo(list, str),
     listify=True,
     default=[],
     validator=_options_validator,