Fix compile error with llvm-15

[masaori335]

I faced the below compile error with LLVM 15. It looks like Ptr.h needs to include <cstddef> explicitly.

In file included from HdrHeap.cc:34:
In file included from ./HdrHeap.h:35:
../../include/tscore/Ptr.h:284:12: error: no type named 'nullptr_t' in namespace 'std'; did you mean simply 'nullptr_t'?
operator==(std::nullptr_t, Ptr<T> const &rhs)
           ^~~~~~~~~~~~~~
           nullptr_t
/usr/local/opt/llvm/include/c++/v1/stddef.h:48:31: note: 'nullptr_t' declared here
    typedef decltype(nullptr) nullptr_t;
                              ^
In file included from HdrHeap.cc:34:
In file included from ./HdrHeap.h:35:
../../include/tscore/Ptr.h:291:12: error: no type named 'nullptr_t' in namespace 'std'; did you mean simply 'nullptr_t'?
operator!=(std::nullptr_t, Ptr<T> const &rhs)
           ^~~~~~~~~~~~~~
           nullptr_t
/usr/local/opt/llvm/include/c++/v1/stddef.h:48:31: note: 'nullptr_t' declared here
    typedef decltype(nullptr) nullptr_t;
                              ^
In file included from HdrHeap.cc:38:
In file included from /opt/src/github.com/apache/trafficserver-asf-master/iocore/eventsystem/I_EventSystem.h:31:
/opt/src/github.com/apache/trafficserver-asf-master/iocore/eventsystem/I_IOBuffer.h:1411:15: error: invalid operands to binary expression ('std::nullptr_t' and 'Ptr<IOBufferBlock>')
  if (nullptr == _head)
      ~~~~~~~ ^  ~~~~~
../../include/tscore/Ptr.h:298:1: note: candidate template ignored: could not match 'const T *' against 'std::nullptr_t'
operator==(T const *lhs, Ptr<T> const &rhs)
^
3 errors generated.
gmake[2]: *** [Makefile:1124: HdrHeap.o] Error 1
gmake[2]: *** Waiting for unfinished jobs....
gmake[2]: Leaving directory '/opt/src/github.com/apache/trafficserver-asf-master/proxy/hdrs'
gmake[1]: *** [Makefile:924: all-recursive] Error 1
gmake[1]: Leaving directory '/opt/src/github.com/apache/trafficserver-asf-master/proxy'
gmake: *** [Makefile:903: all-recursive] Error 1

• LLVM: 15.0.0
• macOS: 12.6
• https://en.cppreference.com/w/cpp/types/nullptr_t

[zwoop]

Cherry-picked to v9.2.x