Change printf attribute to fix clang build

commit e8cd90f0 ("Rewrite gdb_bfd_error_handler") broke the clang
build.

The problem here is that print_error_callback isn't marked as being
printf-like, but it calls string_file::vprintf, triggering:

../../binutils-gdb/gdb/gdb_bfd.c:1202:18: error: format string is not a string literal [-Werror,-Wformat-nonliteral]

This patch applies the attribute to this function.

It also removes the attribute from gdb_bfd_error_handler, because that
function is no longer really printf-like.