Removed crypt dependancy from Makefile for OS X

Fixes crypt library linking for OS X.
>$ make
...
1 warning generated.
cc   dht-example.o dht.o  -lcrypt -o dht-example
ld: library not found for -lcrypt
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [dht-example] Error 1

See https://developer.apple.com/library/mac/documentation/porting/conceptual/portingunix/compiling/compiling.html for more info.
>The linker flag -lcrypt is not supported in OS X.