kconfig: fix memory leak in sym_warn_unmet_dep()

The string allocated in sym_warn_unmet_dep() is never freed, leading
to a memory leak when an unmet dependency is detected.

Fixes: f8f69dc0 ("kconfig: make unmet dependency warnings readable")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Petr Vorel <pvorel@suse.cz>