Here is a typical lkm source structure:

#define MODULE
#define __KERNEL__
#define __KERNE_SYSCALLS__

#include <linux/config.h>
#ifdef MODULE
#include <linux/module.h>
#include <linux/version.h>
#else
#define MOD_INC_USE_COUNT
#define MOD_DEC_USE_COUNT
#endif

#include <linux/types.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/errno.h>
#include <asm/segment.h>
#include <sys/syscall.h>
#include <linux/dirent.h>
#include <asm/unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <ctype.h>

int errno;

char tmp[64];

/* for example, we may need to use ioctl */
_syscall3(int, ioctl, int, d, int, request, unsigned long, arg);

int myfunction(int parm1,char *parm2)
{
   int i,j,k;
   /* ... */
}

int init_module(void)
{
   /* ... */
   printk("\nModule loaded.\n");
   return 0;
}

void cleanup_module(void)
{
   /* ... */
}

Check the mandatory #defines (#define MODULE, #define __KERNEL__) and
#includes (#include <linux/config.h> ...)