gnulib: Fix build of base64 when compiling with memory debugging

gnulib: Fix build of base64 when compiling with memory debugging

When building GRUB with memory management debugging enabled, then the
build fails because of `grub_debug_malloc()` and `grub_debug_free()`
being undefined in the luks2 module. The cause is that we patch
"base64.h" to unconditionaly include "config-util.h", which shouldn't be
included for modules at all. As a result, `MM_DEBUG` is defined when
building the module, causing it to use the debug memory allocation
functions. As these are not built into modules, we end up with a linker
error.

Fix the issue by removing the <config-util.h> include altogether. The
sole reason it was included was for the `_GL_ATTRIBUTE_CONST` macro,
which we can simply define as empty in case it's not set.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>