Fix -Werror=pointer-sign warnings

Fix -Werror=pointer-sign warnings

../../../git/gdb/gnu-nat.c: In function 'gnu_read_inferior':
../../../git/gdb/gnu-nat.c:2282:3: error: pointer targets in passing argument 5 of 'vm_read' differ in signedness [-Werror=pointer-sign]
In file included from /home/yao/Software/hurd-toolchain/bin/../i686-pc-gnu/libc/usr/include/mach.h:38:0,
                 from ./nm.h:23,
                 from ../../../git/gdb/defs.h:500,
                 from ../../../git/gdb/gnu-nat.c:23:
/home/yao/Software/hurd-toolchain/bin/../i686-pc-gnu/libc/usr/include/mach/mach_interface.h:843:15: note: expected 'mach_msg_type_number_t *' but argument is of type 'int *'
../../../git/gdb/gnu-nat.c: In function 'gnu_write_inferior':
../../../git/gdb/gnu-nat.c:2339:4: error: pointer targets in passing argument 5 of 'vm_read' differ in signedness [-Werror=pointer-sign]
In file included from /home/yao/Software/hurd-toolchain/bin/../i686-pc-gnu/libc/usr/include/mach.h:38:0,
                 from ./nm.h:23,
                 from ../../../git/gdb/defs.h:500,
                 from ../../../git/gdb/gnu-nat.c:23:
/home/yao/Software/hurd-toolchain/bin/../i686-pc-gnu/libc/usr/include/mach/mach_interface.h:843:15: note: expected 'mach_msg_type_number_t *' but argument is of type 'int *'

This patch fixes these warnings.

gdb:

2014-02-17  Yao Qi  <yao@codesourcery.com>

	* gnu-nat.c (gnu_read_inferior): Change 'copy_count' type to
	mach_msg_type_number_t.
	(gnu_write_inferior): Likewise.