timers: Keep del_timer[_sync]() exported

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7R8WG



--------------------------------

A previous commit made del_timer[_sync]() obsolete and unexported, which
caused kabi to break. So making del_timer[_sync]() exported, the same as
before.

Signed-off-by: Yu Liao <liaoyu15@huawei.com>