Update wl-mirror to version 0.18.2 / rev 4 via SR 1273198
https://build.opensuse.org/request/show/1273198
by user holzbauer + dimstar_suse
- Update to new upstream release v0.18.2:
* Fix a wlroots compositor warning with wlroots 0.19.0-rc1 and newer when compiling with libdecor
* Fix unused variable warnings when compiling without GBM