e4defrag: fix build failure if libc doesn't have sync_file_range()

e4defrag: fix build failure if libc doesn't have sync_file_range()

The posix_fadvise() to hint to the system that the file can be removed
from memory will probably not work well without the sync_file_range(2)
call, but e4defrag should still fundamentally work, and this will
allow e4defrag to compile if the C library doesn't happen this system
call exposed.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>