printversion: Fix unused variable

printversion: Fix unused variable

clang complains:

    debuginfod.cxx:354:1: error: unused variable 'apba__' [-Werror,-Wunused-const-variable]
    ARGP_PROGRAM_BUG_ADDRESS_DEF = PACKAGE_BUGREPORT;
    ^
    ../lib/printversion.h:47:21: note: expanded from macro 'ARGP_PROGRAM_BUG_ADDRESS_DEF'
      const char *const apba__ __asm ("argp_program_bug_address")
                        ^

The default linkage for consts in C++ is internal, so declare them
extern. While at it, remove the debuginfod workaround for
argp_program_version_hook.

Co-developed-by: Mark Wielaard <mark@klomp.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>