xwayland: Silence format-truncation compilation warnings

We are currently formatting 32-bit signed integers into 8 byte buffers,
which are too small, causing the compiler to complain. Update the buffer
size to the minimum required value of 12 bytes: 1 for the sign, 10 for
the number, and 1 for the null byte terminator.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>