Remove C++ code from libretro.h / Fix compilation with DEBUG=1

[fetzerch]

Compilation with DEBUG=1 failed with:

In file included from src/burner/libretro/tchar.h:7:0, from src/burn/snd/ay8910.c:26: src/burner/libretro/libretro.h:1894:4: error: expected specifier-qualifier-list before ‘retro_input_descriptor’ makefile.libretro:389: recipe for target 'src/burn/snd/ay8910.o' failed

In a debug build, libretro.h is compiled within a C source file, but libretro.h was modified to contain C++ code (constructors).

This change removes the constructors, it's anyways not a good idea to modify libretro.h.

[barbudreadmon]

We should tag @twinaphex and @fr500 about this one since there are changes in libretro.h

[barbudreadmon]

ok, i didn't notice it was a modification specific to this core, i'll merge.