session-config: Correct `config_xml_encoding_bytes_per_char`'s exported type

session-config: Correct `config_xml_encoding_bytes_per_char`'s exported type

The ABI diff reports the following variable type change:

```
1 Changed variable:

  [C] 'const size_t config_xml_encoding_bytes_per_char' was changed to 'std::size_t config_xml_encoding_bytes_per_char' at session
-config.cpp:55:1:
    type of variable changed:
      entity changed from 'const size_t' to 'typedef std::size_t' at c++config.h:310:1
      type size hasn't changed
```

This patch also takes the opportunity to ensure that the "C" linkage
is used for the exported variables.

Change-Id: Id9739d290d1872bc803d2925ffa1d40b04c3e553
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>