Fix pointer assignment with different signedness

Fix pointer assignment with different signedness

This patch fixes these errors below:

../../binutils-gdb/gdb/spu-linux-nat.c: In function ‘spu_symbol_file_add_from_memory’:
../../binutils-gdb/gdb/spu-linux-nat.c:368:3: error: pointer targets in passing argument 2 of ‘spu_proc_xfer_spu’ differ in signedness [-Werror=pointer-sign]
../../binutils-gdb/gdb/spu-linux-nat.c:232:1: note: expected ‘gdb_byte *’ but argument is of type ‘char *’
../../binutils-gdb/gdb/spu-linux-nat.c: In function ‘spu_xfer_partial’:
../../binutils-gdb/gdb/spu-linux-nat.c:598:7: error: pointer targets in passing argument 1 of ‘strtoulst’ differ in signedness [-Werror=pointer-sign]
In file included from ../../binutils-gdb/gdb/defs.h:769:0,
                 from ../../binutils-gdb/gdb/spu-linux-nat.c:21:
../../binutils-gdb/gdb/utils.h:43:15: note: expected ‘const char *’ but argument is of type ‘gdb_byte *’

gdb:

2014-01-08  Yao Qi  <yao@codesourcery.com>

	* spu-linux-nat.c (spu_symbol_file_add_from_memory): Change
	type of 'id' to gdb_byte.  Cast 'id' to 'const char *'.
	(spu_xfer_partial): Cast 'buf' to 'const char *'.