gdb: add ATTRIBUTE_PRINTF to gdb_bfd_error_handler

I see this error when building with clang,

  CXX    gdb_bfd.o
gdb_bfd.c:1180:43: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
  const std::string str = string_vprintf (fmt, ap_copy);
                                          ^~~
1 error generated.

This patch adds missing ATTRIBUTE_PRINTF to fix the error.

Tested on x86_64-linux with gcc 12 and clang 14.