wwan: Replace kernel.h with the necessary inclusions

When kernel.h is used in the headers it adds a lot into dependency hell,
especially when there are circular dependencies are involved.

Replace kernel.h inclusion with the list of what is really being used.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/wwan.h

@@ -4,12 +4,9 @@
 #ifndef __WWAN_H
 #define __WWAN_H
 
-#include <linux/device.h>
-#include <linux/kernel.h>
 #include <linux/poll.h>
-#include <linux/skbuff.h>
-#include <linux/netlink.h>
 #include <linux/netdevice.h>
+#include <linux/types.h>
 
 /**
  * enum wwan_port_type - WWAN port types
@@ -37,6 +34,10 @@ enum wwan_port_type {
 	WWAN_PORT_UNKNOWN,
 };
 
+struct device;
+struct file;
+struct netlink_ext_ack;
+struct sk_buff;
 struct wwan_port;
 
 /** struct wwan_port_ops - The WWAN port operations