debuginfod: Make sure there is only one typedef for debuginfod_client

debuginfod: Make sure there is only one typedef for debuginfod_client

Both debuginfod.h and libdwfl.h have a simple typedef for struct
debuginfod_client. Some compilers pedantically warn when including
both headers that such typedefs are only officially supported in
C11. So guard them with _ELFUTILS_DEBUGINFOD_CLIENT_TYPEDEF to
make them happy.

https://sourceware.org/bugzilla/show_bug.cgi?id=30077

Signed-off-by: Mark Wielaard <mark@klomp.org>