package/ecryptfs-utils: fix musl build

The autobuilders did not catch the error yet because they failed
earlier with other packages, but I am continuing the build based
on the defconfig from:
http://autobuild.buildroot.net/results/6fb/6fbe96af007803acfe9deacf26399cb19e5f2b47/defconfig



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>