Utility: fix compilation on Linux

When compiling on Linux with GCC 4.8, compilation would fail due to the use of
the offsetof macro.  This is a standard macro, defined in stddef.h.  Include
this header to ensure that offsetof is declared.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@206702 91177308-0d34-0410-b5e6-96231b3b80d8