base64: gcc-15 compile issue

[Rudi Heitbaum]

When compiling grub-2.12 with gcc-15-20241208 the following line causes
the following error:

https://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/base64.c#n62

../../grub-core/lib/gnulib/base64.c:65:3:
  error: initializer-string for array of 'char' is too long
  [-Werror=unterminated-string-initialization]

   65 |   "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Regards
Rudi