package/util-linux: fix vipw build with musl

__P() is used for compatibility with old K&R C compilers. With
ANSI C this macro has no effect.

This fixes a compilation error with musl libc because of undeclared
__P.

Fixes:
http://autobuild.buildroot.net/results/237/2377ae85bb9d85ba4c02706207f0b3bde3ccd027



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>