gtypemodule: Cast *_init functions to void(*)(void) first
This is an analogous commit to c1f5e528. The original fix only touched gtype.h and not gtypemodule.h.
The *_init() functions have prototypes incompatible with *InitFunc types they are being cast to. This upsets GCC 8's -Wcast-function-type that's enabled by default with -Wextra.
Let's not have the public header files emit a warning and neutralize it by doing a void(*)(void) cast first.
Signed-off-by: Philip Withnall firstname.lastname@example.org
Closes #1666 (closed)