[Xcb] Fix #23403 - compiler padding causes reply parsing to use incorrect offsets

[Daniel Martin]

Hi,

I just had time to have a closer look at bug #23403. Here's my proposal
to fix it:

[PATCH proto 1/3] Add optional packed attribute to struct elements
* Add the new attribute in the doc, schema and handle it in xtypes.py.

[PATCH lib] c_client.py: Handle opt. packed attr. in structs
* Handle the new attribute, add __attribute__((packed)) if requested.

[PATCH proto 2/3] sync: Use packed attribute for SYSTEMCOUNTER
* Use the new attribute and fix #23403.

[PATCH proto 3/3] sync: Fix length of name in SYSTEMCOUNTER
* Fix side effect which arised.