btrfs-progs: makefile: drop u option from ar invocation

btrfs-progs: makefile: drop u option from ar invocation

In newer distros (ubuntu 15.10, fedora rawhide) the binutils
ar uses the new D flag per default to build deterministic
binaries.
Without this patch the following warning is issued, when
building btrfs-progs:

    [AR]     libbtrfs.a
/usr/bin/ar: `u' modifier ignored since `D' is the default (see `U')

For libtrfs.a performance benefit of the u option can be neglected,
so drop the u option and silence the warning.

In the future one might want to explicitly add the D option anyway.

Signed-off-by: Arnd Hannemann <arnd@arndnet.de>
Signed-off-by: David Sterba <dsterba@suse.com>