Use gdb::byte_vector in target_write_memory
    
    Pedro pointed out that target_write_memory should use gdb::byte_vector
    rather than std::vector<unsigned char>.
    
    gdb/gdbserver/ChangeLog
    2019-08-15  Tom Tromey  <tromey@adacore.com>
    
    	* target.c (target_write_memory): Use gdb::byte_vector.