libcheese: Add GtkWidget cast to avoid an incompatible-pointer-types error

This is required for compilation with GCC 14.

Full GCC 14 compatibility needs the Vala compiler kludge from vala!369 (merged).

Related to:

• https://fedoraproject.org/wiki/Changes/PortingToModernC
• https://fedoraproject.org/wiki/Toolchain/PortingToModernC