kconfig: fix warning

In file included from scripts/kconfig/zconf.tab.c:2502:
scripts/kconfig/expr.c:1033: warning: no previous prototype for 'expr_simplify_unmet_dep'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>