xdg_shell_v6: add _POSIX_C_SOURCE define for struct timespec

wlroots 1e0e73 added struct timespec to wlr_seat.h, so we need to
define a _POSIX_C_SOURCE large enough whenever we include wlr_seat.h

sway/desktop/xdg_shell_v6.c

@@ -1,3 +1,4 @@
+#define _POSIX_C_SOURCE 199309L
 #include <stdlib.h>
 #include <wayland-server.h>
 #include <wlr/types/wlr_xdg_shell_v6.h>