graphics/frogr: fix build with GCC-based architectures

/usr/local/include/libsoup-2.4/libsoup/soup-session-async.h: In function '_soup_session_async_get_type_undeprecated':
/usr/local/include/libsoup-2.4/libsoup/soup-session-async.h:44: warning: 'soup_session_async_get_type' is deprecated (declared at /usr/local/include/libsoup-2.4/libsoup/soup-session-async.h:38)

With GCC8, it doesn't cause a warning (and error, because -Werror).

Approved by: mentors (implicit approval)