sysvinit: make sure to link against libcrypt

sysvinit Makefile checks if /usr/lib/libcrypt.a exists to determine
whether it should link against libcrypt or not. This test fails on
distributions such as Fedora, in which /usr/lib/libcrypt.a does not
exist (it is in /usr/lib64), but where libcrypt is available on the
target. As libcrypt is available in both uClibc and gclibc, we just
force sysvinit to link against libcrypt.

Fixes bug #2401.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>