l2tp: fix export of header file for userspace

The header file l2tp.h should be exported to the installed include/linux/
tree for userspace programs.

This patch fixes compilation errors in L2TP userspace apps which want to
use the new L2TP support introduced in 2.6.35.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/Kbuild

@@ -276,6 +276,7 @@ ifneq ($(wildcard $(srctree)/arch/$(SRCARCH)/include/asm/kvm_para.h \
 		  $(srctree)/include/asm-$(SRCARCH)/kvm_para.h),)
 unifdef-y += kvm_para.h
 endif
+unifdef-y += l2tp.h
 unifdef-y += llc.h
 unifdef-y += loop.h
 unifdef-y += lp.h