Fix invalid "size-changed" callback function declaration

The function "void sensors_set_size(...)" used as a "size-changed" plugin callback was being incorrectly cast to "gboolean callback(...)" which in case of the AMD64 ABI resulted in the callback() returning whatever was in the %rax register after the last statement in the body of "void sensors_set_size()" was executed.

Closes: #15 (closed)

Closes: !17 (closed)

Thanks: Stergios Gemelas @stergem