Fix loading of RSS Guard plugins

[guihkx]

Marking as draft since I'll have to test the build.

I'll merge later if the fix works.

Fixes #73

[flathubbot]

Started test build 132235

[flathubbot]

Build 132235 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/115240/io.github.martinrotter.rssguard.flatpakref

[guihkx]

Works for me:

$ flatpak run io.github.martinrotter.rssguard//test
[...]
time="     0.807" type="debug" -> core: Checking for plugins in '/app/bin/../lib/rssguard'.
time="     0.813" type="debug" -> core: Plugin '/app/lib/rssguard/librssguard-standard.so' loaded.
time="     0.815" type="debug" -> core: Plugin '/app/lib/rssguard/librssguard-nextcloud.so' loaded.
time="     0.818" type="debug" -> core: Plugin '/app/lib/rssguard/librssguard-greader.so' loaded.
time="     0.821" type="debug" -> core: Plugin '/app/lib/rssguard/librssguard-feedly.so' loaded.
time="     0.823" type="debug" -> core: Plugin '/app/lib/rssguard/librssguard-ttrss.so' loaded.
time="     0.826" type="debug" -> core: Plugin '/app/lib/rssguard/librssguard-gmail.so' loaded.

Let's merge this quick so the breakage affects as little people as possible...