seq: tstamp: use wrapper structure instead of union

seq: tstamp: use wrapper structure instead of union

In UAPI for ALSA Sequencer, snd_seq_timestamp is union with
snd_seq_tick_time_t and struct snd_seq_real_time types. The
libalsaseq has GLib Boxed object for the union, named as
ALSASeqTstamp.

```
$ cat build/src/seq/ALSASeq-0.0.gir
    ...
    <union name="Tstamp"
           c:type="ALSASeqTstamp"
           glib:type-name="ALSASeqTstamp"
           glib:get-type="alsaseq_tstamp_get_type"
           c:symbol-prefix="tstamp">
      <source-position filename="../src/seq/tstamp.h" line="14"/>
      ...
    </union>
    ...
```

Although this is valid in a view of GObject
Introspection, it's not nesessarily convenient to language
bindings because some programming language doesn't support
union type. In this case, the type is not available.

This commit adds an alternative structure to wrap the union.
As a result, ALSASeqTstamp is GLib Boxed object for the
wrapper structure.

```
$ cat build/src/seq/ALSASeq-0.0.gir
    ...
    <record name="Tstamp"
            c:type="ALSASeqTstamp"
            glib:type-name="ALSASeqTstamp"
            glib:get-type="alsaseq_tstamp_get_type"
            c:symbol-prefix="tstamp">
      <source-position filename="../src/seq/tstamp.h" line="18"/>
      <field name="tstamp" writable="1">
        <type name="gpointer" c:type="snd_seq_timestamp"/>
      </field>
      ...
    </record>
    ...
```

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>