resource: move kernel function inside __KERNEL__

It is an internal function. Move it inside __KERNEL__ ifdef, along
with task_struct declaration.

Then we get:
--- /usr/include/linux/resource.h       2009-09-14 15:09:29.000000000 +0200
+++ usr/include/linux/resource.h       2010-01-04 11:30:54.000000000 +0100
@@ -3,8 +3,6 @@

 #include <linux/time.h>

-struct task_struct;
-
 /*
  * Resource control/accounting header file for linux
  */
@@ -70,6 +68,5 @@
  */
 #include <asm/resource.h>

-int getrusage(struct task_struct *p, int who, struct rusage *ru);

 #endif

***********

include/linux/Kbuild is untouched, since unifdef is run even on
headers-y nowadays.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>