use gdb_byte in ada-lang.c::ada_value_primitive_packed_val...

use gdb_byte in ada-lang.c::ada_value_primitive_packed_val...

... instead of "unsigned char".

gdb/Changelog:

        * ada-lang.c (ada_value_primitive_packed_val): Change the type
        of local variables src and unpacked to "gdb_type *" instead of
        "unsigned char *".