ccnl-pkt: use random_uint32() instead of rand() for RIOT

[cgundogan]

Contribution description

In RIOT, the correct way to generate a 4 byte random number is with using random_uint32().

[PeterKietzmann]

semaphoreci doesn't like that formatting:

ccnl-pkt-builder.c:203:5: error: missing braces around initializer [-Werror=missing-braces]
     ccnl_interest_opts_u default_opts = { 0 };