graphics/wayland: Remove libm from pkgconfig Libs
This changes seems to confuse CMake for some unknown reason so for now
remove libm from the Libs section, it's in the Libs.private so it's picked up
just fine.
Fixes: 9e3878cf8428 ("graphics/wayland: Update to 1.24.0")
Sponsored by: Beckhoff Automation GmbH & Co. KG