meson: add -Wimplicit-fallthrough=3 to compilation options (#7393)

At some point before gcc-7 was released, -Wimplicit-fallthrough=3 was included
in -Wextra. The documentation for gcc-7.2.1-2.fc27.x86_64 still says that, but
empirical testing shows that it's not. The documentation also misstates that
-Wimplicit-fallthrough is equivalent to -Wimplicit-fallthrough=3.

Let's add -Wimplicit-fallthrough=3 explicitly to get the warnings if we regress.

Prompted by #7389.