st/oxr: fix compiler warnings for XrStructureType name lengths >= XR_MAX_STRUCTURE_NAME_SIZE
OpenXR 1.1.48 added the extension XR_META_simultaneous_hands_and_controllers,
this ext adds new XrStructureType names that are greater than
XR_MAX_STRUCTURE_NAME_SIZE
(it should extend XR_KHR_extended_struct_name_lengths but currently does not),
causing snprintf warning messages in oxr_xrStructureTypeToString
Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2479>