m4: disable gcc8 -Wcast-function-type warnings from -Wextra

m4: disable gcc8 -Wcast-function-type warnings from -Wextra

The -Wextra flag bundle gained a new warning -Wcast-function-type.
This complains if you cast between two function prototypes where
the number of parameters or their data types are not compatible.
Unfortunately we need such "bad" function casts for our event
callbacks. It is possible to silence the warning by first casting
to the generic "void (*)(void)" function prototype, but that is
rather ugly to add throughout libvirt code.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>