[lldb] on s390x fix override issue

Summary:
This fixes an override issue by marking a function as const so that the
signature maps to the signature of the function in the base class.

This is the original error:

In file included from /root/llvm/lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_s390x.cpp:11:
/root/llvm/lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_s390x.h:79:10: error: 'size_t lldb_private::process_linux::NativeRegisterContextLinux_s390x::GetGPRSize()' marked 'override', but does not override
   79 |   size_t GetGPRSize() override { return sizeof(m_regs); }
      |          ^~~~~~~~~~

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D83580