server: Extend display name string size

Typically this is a number between 0 and 32. Just that the compiler doesn't know that well. Make the string buffer a bit larger, so that it fits the longer integers. Fixes build warnings like:

../subprojects/wayland/src/wayland-server.c: In function ‘wl_display_add_socket_auto’:
../subprojects/wayland/src/wayland-server.c:1649:70: error: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size 8 [-Werror=format-truncation=]
 1649 |                 snprintf(display_name, sizeof display_name, "wayland-%d", displayno);
      |                                                                      ^~
../subprojects/wayland/src/wayland-server.c:1649:61: note: directive argument in the range [-2147483647, 32]
 1649 |                 snprintf(display_name, sizeof display_name, "wayland-%d", displayno);
      |                                                             ^~~~~~~~~~~~
../subprojects/wayland/src/wayland-server.c:1649:17: note: ‘snprintf’ output between 10 and 20 bytes into a destination of size 16
 1649 |                 snprintf(display_name, sizeof display_name, "wayland-%d", displayno);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Seen in GTK CI.