Drop server.h include from input/input-manager.h

The only reason it's included there is for a declaration of
struct sway_server, but we can just forward-declare it.

This avoids rebuilding almost all of Sway when touching server.h.
All other server.h includes are from source files, not headers.

include/sway/criteria.h

@@ -7,6 +7,10 @@
 #include "list.h"
 #include "tree/view.h"
 
+#if WLR_HAS_XWAYLAND
+#include "sway/xwayland.h"
+#endif
+
 enum criteria_type {
 	CT_COMMAND                 = 1 << 0,
 	CT_ASSIGN_OUTPUT           = 1 << 1,

include/sway/input/input-manager.h

@@ -5,10 +5,11 @@
 #include <wlr/types/wlr_virtual_keyboard_v1.h>
 #include <wlr/types/wlr_virtual_pointer_v1.h>
 #include <wlr/types/wlr_transient_seat_v1.h>
-#include "sway/server.h"
 #include "sway/config.h"
 #include "list.h"
 
+struct sway_server;
+
 struct sway_input_device {
 	char *identifier;
 	struct wlr_input_device *wlr_device;