misc/openvdb: fix build on GCC architectures

/wrkdirs/usr/ports/misc/openvdb/work/openvdb-7.1.0/openvdb/points/AttributeSet.h:461:60: error: 'CHAR_BIT' was not declared in this scope

461 |     static size_t groupBits() { return sizeof(GroupType) * CHAR_BIT; }
    |                                                            ^~~~~~~~

/wrkdirs/usr/ports/misc/openvdb/work/openvdb-7.1.0/openvdb/points/AttributeSet.h:1:1: note: 'CHAR_BIT' is defined in header '<climits>'; did you forget to '#include <climits>'?

+++ |+#include <climits>
  1 | // Copyright Contributors to the OpenVDB Project