add linker option for libcrypto

Got an error when building dnc on Archlinux:

    Linking C executable dnc
    /usr/bin/ld: CMakeFiles/dnc.dir/dnc.c.o: undefined reference to symbol 'X509_get_subject_name'
    /usr/bin/ld: note: 'X509_get_subject_name' is defined in DSO /usr/lib/libcrypto.so.1.0.0 so try adding  it to the linker command line
    /usr/lib/libcrypto.so.1.0.0: could not read symbols: Invalid operation
    collect2: error: ld returned 1 exit status

Adding the -lcrypto flag solved it.