kconfig: Fix compiler warning in menu.c

This fixes the following compiler warning:

In file included from scripts/kconfig/zconf.tab.c:2537:0:
scripts/kconfig/menu.c: In function ‘get_symbol_str’:
scripts/kconfig/menu.c:590:18: warning: ‘jump’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     jump->offset = strlen(r->s);
                  ^
In file included from scripts/kconfig/zconf.tab.c:2537:0:
scripts/kconfig/menu.c:551:19: note: ‘jump’ was declared here
  struct jump_key *jump;

Signed-off-by: Hans de Goede <hdegoede@redhat.com>