Fix "extra ';' outside of a function" warning (#3929)

Fix the following warning seen with clang:

include/pybind11/detail/smart_holder_poc.h:109:2: error: extra ';' outside of a function is incompatible with C++98 [-Werror,-Wc++98-compat-extra-semi]

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>