asn1: patch ITS timestamp for 32-bit targets (see #245)

[khevessy]

Tried to implement aforementioned fix based on https://github.com/riebl/vanetza/blob/master/vanetza/asn1/patches/security-64bit-types.patch - as advised by @riebl in #245.

Raphael, please, I am not sure what does the asn_INTEGER_specifics_t structure mean, does it also have to be added here?

[riebl]

Thanks for your work, Karel! Yes, I think the the asn_INTEGER_specifics_t structure is necessary as the encoding and decoding otherwise defaults to "signed integer", see

vanetza/vanetza/asn1/support/INTEGER_uper.c

Line 61 in ba4d1fb

if(specs && specs->field_unsigned) {

So from all these "specifics", the field_unsigned flag is important for us in this case.

[khevessy]

Thanks for the explanation. Added the structure now.