meson: add -lm to gstreamer-check-1.0 pkgconfig file
meson: add -lm to gstreamer-check-1.0 pkgconfig file

Fixes warning with meson from git about LIBM not being
defined in the configuration_data.

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer