[lldb] Fix warning in MinidumpFileBuilder.cpp

Fixes the following warning:

$llvm_project/lldb/source/Plugins/ObjectFile/Minidump/MinidumpFileBuilder.cpp:744:11: warning:
format specifies type 'long' but the argument has type 'lldb::offset_t' (aka 'unsigned long long') [-Wformat]
          m_data.GetByteSize());
          ^~~~~~~~~~~~~~~~~~~~