Unbreak mips native build

Unbreak mips native build

When I build mips native gdb today, I get the follow error,

../../../git/gdb/mips-linux-nat.c: In function '_initialize_mips_linux_nat':
../../../git/gdb/mips-linux-nat.c:792:15: error: assignment from incompatible pointer type [-Werror]
cc1: all warnings being treated as errors

It looks an oversight of recent target_ops delegation patches.  This
patch is to fix the build error.

gdb:

2014-02-24  Yao Qi  <yao@codesourcery.com>

	* mips-linux-nat.c (super_close): Update its type.
	(mips_linux_close): Pass 'self' to super_close.