fix build error in ‘vfs_preallocate’: vfs.c:613:12: error: ‘dest_desc’ undeclared (first use in this function)

When platform does not define HAVE_POSIX_FALLOCATE we get this error:

  CC     vfs.lo
../../../../mc/lib/vfs/vfs.c: In function 'vfs_preallocate':
../../../../mc/lib/vfs/vfs.c:613:12: error: 'dest_desc' undeclared (first use in this function)
../../../../mc/lib/vfs/vfs.c:613:12: note: each undeclared identifier is reported only once for each function it appears in
../../../../mc/lib/vfs/vfs.c:610:22: warning: unused parameter 'dest_vfs_fd' [-Wunused-parameter]

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

lib/vfs/vfs.c

@@ -610,7 +610,7 @@ int
 vfs_preallocate (int dest_vfs_fd, off_t src_fsize, off_t dest_fsize)
 {
 #ifndef HAVE_POSIX_FALLOCATE
-    (void) dest_desc;
+    (void) dest_vfs_fd;
     (void) src_fsize;
     (void) dest_fsize;
     return 0;