deskutils/spice-gtk: fix build on GCC architectures
Use C11 compiler:
../common/demarshallers.h:25: error: previous declaration of 'message_destructor_t' was here
generated_server_demarshallers.c:164: error: redefinition of typedef 'spice_parse_channel_func_t'
../common/demarshallers.h:29: error: previous declaration of 'spice_parse_channel_func_t' was here
Approved by: mentors (implicit approval)