misc/openvdb: fix build on GCC architectures

Compiling on GCC architectures fails:
/wrkdirs/usr/ports/misc/openvdb/work/openvdb-6.1.0/openvdb/points/AttributeSet.cc: In member function 'openvdb::v6_1::points::AttributeSet::Descriptor::Ptr openvdb::v6_1::points::AttributeSet::Descriptor::duplicateDrop(const std::vector<long unsigned int>&) const':
/wrkdirs/usr/ports/misc/openvdb/work/openvdb-6.1.0/openvdb/points/AttributeSet.cc:875:59: error: 'CHAR_BIT' was not declared in this scope

875 |             const size_t GROUP_BITS = sizeof(GroupType) * CHAR_BIT;
    |                                                           ^~~~~~~~

/wrkdirs/usr/ports/misc/openvdb/work/openvdb-6.1.0/openvdb/points/AttributeSet.cc:35:1: note: 'CHAR_BIT' is defined in header '<climits>'; did you forget to '#include <climits>'?

 34 | #include "AttributeGroup.h"
+++ |+#include <climits>
 35 |

While here, add USES=xorg for TOOLS option.

PR: 240630
Approved by: linimon (mentor), yuri (maintainer timeout)