mdadm: fix compilation with musl 1.2.4

mdadm: fix compilation with musl 1.2.4

_LARGEFILE64_SOURCE has to be defined in the source, or CFLAGS can be used
to pass -D_LARGEFILE64_SOURCE to allow to keep using LFS64 definitions.

Fixes error in the form of:
restripe.c: In function 'restore_stripes':
restripe.c:758:43: error: 'off64_t' undeclared (first use in this function); did you mean 'off_t'?
  758 |                                          (off64_t)read_offset) {
      |                                           ^~~~~~~
      |                                           off_t
restripe.c:758:43: note: each undeclared identifier is reported only once for each function it appears in
restripe.c:758:51: error: expected ')' before 'read_offset'
  758 |                                          (off64_t)read_offset) {
      |                                                   ^~~~~~~~~~~
      |                                                   )
restripe.c:757:36: note: to match this '('
  757 |                                 if (lseek64(source, read_offset, 0) !=
      |                                    ^
make[3]: *** [Makefile:193: restripe.o] Error 1

Fixes: fff878c5bcda ("toolchain/musl: update to 1.2.4")
Signed-off-by: Nick Hainke <vincent@systemli.org>