lib/blkid: suppress -Wunused-result warning in blkid_flush_cache()

lib/blkid: suppress -Wunused-result warning in blkid_flush_cache()

When _FORTIFY_SOURCE is defined, glibc annotates link() with the
warn_unused_result function attribute.  With gcc, that makes
'(void) link()' cause a -Wunused-result warning, despite the explicit
cast to void.  That's annoying, since the use case in lib/blkid/save.c
is legitimate (opportunistic backup).  So let's suppress this warning.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>