trafgen: parser: fix regression on last pkt element

Jon Schipp reported that a arbitrary packet config
such as ...

{
 "MyPacket",
 "MyPacket",
 "MyPacket",
 "MyPacket",
 "MyPacket",
 "MyPacket",
 "MyPacket",
 "MyPacket",
}

... doesn't work anymore, although it used to work. This
regression was introduced by commit f9591eebe40c ("trafgen:
lexer/parser: fix cpu() selection and whitespacing").

Reported-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>