ci: Remove --werror

ci: Remove --werror

Turns out that GCC errors out when building the GLib test suite, as it
now checks for overflows in allocator functions, and we're testing for
those.

This would not be an issue for GTK, but since we're building GLib as a
subproject, we get failures for those as well.

Until we can find out how to disable errors for subprojects, or fix the
GLib test suite not to trip up warnings in GCC, we're going to live
without compiler warnings treated as errors for a while.