xfsprogs: Fix building xfsprogs on 32-bit platforms (again)

xfsprogs: Fix building xfsprogs on 32-bit platforms (again)

Building xfsprogs on 32-bit platforms was broken again by the recent
split of BUILD_CFLAGS from CFLAGS.  -D_FILE_OFFSET_BITS=64 was not added
to BUILD_CFLAGS, but in fact BUILD_CFLAGS is used to compile
crc32selftest, which includes xfs.h and therefore requires this
declaration.  Fix this by adding -D_FILE_OFFSET_BITS=64 to BUILD_CFLAGS.

Fixes: 0a71e3839630 ("build: Allow compiling xfsprogs in a cross compile environment")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>