If ast_channel struct is not declared before ast_register_application2,

If ast_channel struct is not declared before ast_register_application2,
gcc thinks that the implicit declaration in module.h is different from
the one in channel.h, and issues a warning about incompatible pointer
types. A forward declaration before including module.h fixes this.

Due to some brain-deadness in Ast, including channel.h before module.h
causes the compilation fail altogether.

Signed-off-by: Andreas Eversberg <jolly@eversberg.eu>