[PLUGINS] Add 'extern "C"' wrappers to several header files

[Duncan Sands]

Hi Diego, thanks for your comments.

> Ian, wouldn't you have needed this for --enable-build-with-cxx?

I wasn't aware of -enable-build-with-cxx.  I just tried it and
unfortunately my patch breaks the build when it is enabled.
Compiling insn-attrtab.c:

In file included from insn-attrtab.c:15:
insn-attr.h:50: error: previous declaration of ‘int 
get_attr_length(rtx_def*)’ with ‘C++’ linkage
output.h:50: error: conflicts with new declaration with ‘C’ linkage
insn-attr.h:51: error: previous declaration of ‘void 
shorten_branches(rtx_def*)’ with ‘C++’ linkage
output.h:58: error: conflicts with new declaration with ‘C’ linkage

I'm not sure what the best way to proceed is.

Ciao,

Duncan.