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.

sway/input/cursor.c

@@ -25,6 +25,7 @@
 #include "sway/layers.h"
 #include "sway/output.h"
 #include "sway/scene_descriptor.h"
+#include "sway/server.h"
 #include "sway/tree/container.h"
 #include "sway/tree/root.h"
 #include "sway/tree/view.h"

sway/input/keyboard.c

@@ -13,6 +13,7 @@
 #include "sway/input/seat.h"
 #include "sway/input/cursor.h"
 #include "sway/ipc-server.h"
+#include "sway/server.h"
 #include "log.h"
 
 #if WLR_HAS_SESSION

sway/input/seatop_default.c

@@ -11,6 +11,7 @@
 #include "sway/input/tablet.h"
 #include "sway/layers.h"
 #include "sway/output.h"
+#include "sway/server.h"
 #include "sway/scene_descriptor.h"
 #include "sway/tree/view.h"
 #include "sway/tree/workspace.h"

sway/input/switch.c

@@ -1,5 +1,6 @@
 #include "sway/config.h"
 #include "sway/input/switch.h"
+#include "sway/server.h"
 #include "log.h"
 
 struct sway_switch *sway_switch_create(struct sway_seat *seat,

sway/input/tablet.c

@@ -7,6 +7,7 @@
 #include "sway/input/cursor.h"
 #include "sway/input/seat.h"
 #include "sway/input/tablet.h"
+#include "sway/server.h"
 
 #if WLR_HAS_LIBINPUT_BACKEND
 #include <wlr/backend/libinput.h>

sway/input/text_input.c

@@ -9,6 +9,8 @@
 #include "sway/input/text_input.h"
 #include "sway/input/text_input_popup.h"
 #include "sway/layers.h"
+#include "sway/server.h"
+
 static void input_popup_update(struct sway_input_popup *popup);
 
 static struct sway_text_input *relay_get_focusable_text_input(