sspi dosn't print any debug log when run mptcpd with --debug options

[andywu106]

Describe the bug I run mptcpd by: /usr/local/libexec/mptcpd --debug, and create a mptcp connection, sspi dosn't print any debug logs，but sspi_send_addrs() call l_debug() to print debug log.

To Reproduce Steps to reproduce the behavior:

1. set "path-manager=sspi" in /usr/local/etc/mptcpd/mptcpd.conf
2. run mptcpd by: /usr/local/libexec/mptcpd --debug
3. create a new mptcp connection.

Desktop (please complete the following information):

• Linux Kernel: [5.12]
• mptpd: [0.7]

[ossama-othman]

This problem used to be caused by a problem in ELL that prevented l_debug() from working in dynamically linked code as discussed in the "Debug logging with ELL" thread on the ELL mailing list, but was later fixed by this commit in ELL.

Presumably this is a regression that was introduced when mptcpd re-implemented its plugin framework to work around the l_plugin API that was dropped from ELL.