Silence warnings about incompatible plugins.

Silence warnings about incompatible plugins.

I have been looking at a Fedora bug report[1] from a user who was
receiving warning messages from the BFD library about incompatible
plugins.  It turns out that they had both 32-bit and 64-bit versions
of the same plugin installed, and the BFD library was attempting to
load all of them.

After thinking about it for a while, it seemed to me that the simplest
solution was to not warn about incompatible plugins whilst attempting
to create a list of viable plugins.

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1836618

	* plugin.c (try_load_plugin): Suppress the error message about
	being unable to open a plugin if creating a list of viable
	plugins.