net/nepenthes: unbreak with gcc6 or later

In file included from SMBDialogue.cpp:34:0:
asn1-shellcodes.h:25:1: error: narrowing conversion of '133' from 'int' to 'char' inside { } [-Wnarrowing]
};
^
asn1-shellcodes.h:25:1: error: narrowing conversion of '255' from 'int' to 'char' inside { } [-Wnarrowing]
asn1-shellcodes.h:25:1: error: narrowing conversion of '200' from 'int' to 'char' inside { } [-Wnarrowing]
asn1-shellcodes.h:300:1: error: narrowing conversion of '191' from 'int' to 'char' inside { } [-Wnarrowing]
};
^
[...]