compress_ops: add magic and look for compress_plugin_mops symbol

compress_ops: add magic and look for compress_plugin_mops symbol

We need more validation of passed ops structure to allow
further API changes in future.

This change looks for compress_plugin_mops symbol in the
dynamic plugin library to make sure that the new magic
and new get_tstamp64 members are handled correctly.

Closes: https://github.com/alsa-project/tinycompress/pull/31
Link: https://github.com/alsa-project/tinycompress/pull/29
Link: https://github.com/alsa-project/tinycompress/pull/30
Fixes: 0bd5530 ("compress_ops: add get_tstamp64")
Signed-off-by: Jaroslav Kysela <perex@perex.cz>