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: f8f69dc0b4e0 ("kconfig: make unmet dependency warnings readable")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Petr Vorel <pvorel@suse.cz>

scripts/kconfig/symbol.c

@@ -388,6 +388,7 @@ static void sym_warn_unmet_dep(const struct symbol *sym)
 			       "  Selected by [m]:\n");
 
 	fputs(str_get(&gs), stderr);
+	str_free(&gs);
 	sym_warnings++;
 }