module/i3: use our own copy of i3/ipc.h

The main advantage is when targeting sway/wayland instead of i3/x11;
sway doesn't package ipc.h. There *is* an ipc.h in the sway sources,
but it doesn't define e.g the header format, and has different names
for the constants.

modules/i3-common.c

@@ -12,7 +12,6 @@
  #include <xcb/xcb_aux.h>
 #endif
 
-#include <i3/ipc.h>
 #include <json-c/json_tokener.h>
 
 #define LOG_MODULE "i3:common"
@@ -23,6 +22,8 @@
  #include "../xcb.h"
 #endif
 
+#include "i3-ipc.h"
+
 #if defined(ENABLE_X11)
 static bool
 get_socket_address_x11(struct sockaddr_un *addr)