check_ldap: Fix warning implicit-function-declaration function ‘ldap_check_cert’

See,
check_ldap.c: In function ‘main’:
check_ldap.c:191:11: warning: implicit declaration of function ‘ldap_check_cert’; did you mean ‘ldap_sync_init’? [-Wimplicit-function-declaration]
    return ldap_check_cert(ld);
           ^~~~~~~~~~~~~~~
           ldap_sync_init

plugins/check_ldap.c

@@ -49,6 +49,7 @@ enum {
 
 int process_arguments (int, char **);
 int validate_arguments (void);
+int ldap_check_cert (LDAP *ld);
 void print_help (void);
 void print_usage (void);