[fix] added ldconfig

Without ldconfig invocation, the installed libsodium library cannot be
found right after install, resulting in cryptic error messages like:

	cffi.ffiplatform.VerificationError: importing '[...]/pysodium/__pycache__/_cffi__x33f03c4bx6f2621b3.x86_64-linux-gnu.so':	libsodium.so.4: cannot open shared object file: No such file or directory