Fix the build for FEATURE_GDBJIT

[cshung]

I tried to play around with FEATURE_GDBJIT, without this change, it doesn't even compile.

This fixed the build, but debugging with gdb is still not working, not sure what is wrong.

[AaronRobinsonMSFT]

Can you share the build failures you are seeing?

[cshung]

Can you share the build failures you are seeing?

Sure, here it is:

  In file included from /home/andrewau/git/runtime/src/coreclr/vm/prestub.cpp:45:
  /home/andrewau/git/runtime/src/coreclr/vm/customattribute.h:13:11: error: redefinition of 'Attribute' as different kind of symbol
  namespace Attribute
            ^
  /home/andrewau/git/runtime/src/coreclr/vm/../inc/llvm/Dwarf.h:125:6: note: previous definition is here
  enum Attribute : uint16_t {