fix possible segmentation fault without symbol

As Jovi noticed:
    Segmentation fault for "trace sdt:/lib64/libc.so.6 {...}"
    No symbol given in that case.

Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>
check libelf dependency in Makefile

If no libelf found, then NO_LIBELF will set, and
disable symbol resolving in ktap(like uprobe and sdt).

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>