rpicamsrc: meson: drop map file and fix plugin symbol export with newer gstreamer...

rpicamsrc: meson: drop map file and fix plugin symbol export with newer gstreamer versions

Use -fvisibility instead of a map file for symbol export, so that
the right symbols get exported with newer gstreamer versions. Older
GStreamer versions also still work of course.

Fixes blacklisting/plugin-loading issues with GStreamer >= 1.14

Fixes https://github.com/thaytan/gst-rpicamsrc/issues/984,
closes https://github.com/thaytan/gst-rpicamsrc/issues/94
and https://github.com/thaytan/gst-rpicamsrc/issues/67