kconfig: fix memory leak in sym_warn_unmet_dep()

[ Upstream commit a409fc1463d664002ea9bf700ae4674df03de111 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>