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>

scripts/kconfig/expr.h

@@ -206,6 +206,7 @@ struct expr *expr_extract_eq_and(struct expr **ep1, struct expr **ep2);
 struct expr *expr_extract_eq_or(struct expr **ep1, struct expr **ep2);
 void expr_extract_eq(enum expr_type type, struct expr **ep, struct expr **ep1, struct expr **ep2);
 struct expr *expr_trans_compare(struct expr *e, enum expr_type type, struct symbol *sym);
+struct expr *expr_simplify_unmet_dep(struct expr *e1, struct expr *e2);
 
 void expr_fprint(struct expr *e, FILE *out);
 struct gstr; /* forward */