Change type of write_qxfer_response parameter

Change type of write_qxfer_response parameter

Fixes:

/home/simark/src/binutils-gdb/gdb/gdbserver/server.c: In function ‘int write_qxfer_response(char*, const void*, int, int)’:
/home/simark/src/binutils-gdb/gdb/gdbserver/server.c:398:32: error: invalid conversion from ‘const void*’ to ‘const gdb_byte* {aka const unsigned char*}’ [-fpermissive]
           &out_len, PBUFSIZ - 2) + 1;
                                ^
gdb/gdbserver/ChangeLog:

	* server.c (write_qxfer_response): Change type of data to
	gdb_byte *.