Fix compiler warnings caused by GValue initializers

Just use G_VALUE_INIT everywhere and also use it unconditionally
as we depend on GLib >= 2.30 now.

snra-manager.c: In function 'make_player_clients_list_msg':
snra-manager.c:205: warning: missing initializer
snra-manager.c:205: warning: (near initialization for 'p.data')
snra-manager.c:216: warning: missing initializer
snra-manager.c:216: warning: (near initialization for 'tmp.data')
Merge pull request #6 from sdroege/master

Another compiler warning fix