package/pure-ftpd: add optional support for libsodium

When libsodium was compiled before, pure-ftpd will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/pure-statsdecode | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libsodium.so.17]
 0x0000000000000001 (NEEDED)             Shared library: [libcrypt.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]

The build system offers no option to en-/disable libsodium support,
check configure.ac, line 953.

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