meson: Let the compiler do it's job

Don't disable all warnings.
1 job for meson in 1 minute and 56 seconds (queued for 1 second)
Name Stage Failure
failed
build-debian-gcc Build
   HdyLeaflet         *hdy_leaflet;
^~~~~~~~~~~
../src/chatty-window.c:158:23: error: unused variable ‘vbox’ [-Werror=unused-variable]
GtkBox *vbox;
^~~~
cc1: all warnings being treated as errors
[7/16] Compiling C object 'src/src@@chatty@exe/meson-generated_.._chatty-resources.c.o'.
ninja: build stopped: subcommand failed.
ERROR: Job failed: exit code 1