dtls_time: use 32 bit int for time
dtls_time: use 32 bit int for time

On a 16 bit system a 16 bit int would be used for the time and this is
too small. In cooja on a wismote target I had an overflow every some
minutes.

32bit for the time should work for some years.

Signed-off-by: Hauke Mehrtens <435ddd46dc66c007a1fa20144c823d37e0d23436@hauke-m.de>

spark/tinydtls,yunus/TinyDTLS-WebID,jvermillard/tinydtls,spark/tinydtls,fvdnabee/tinydtls,thecodemaiden/tinydtls-fork,yunus/TinyDTLS-WebID,nmacs/tinydtls,jvermillard/tinydtls,nmacs/tinydtls,thecodemaiden/tinydtls-fork,yunus/TinyDTLS-WebID,OlegHahm/tinydtls,spark/tinydtls,OlegHahm/tinydtls,fvdnabee/tinydtls,OlegHahm/tinydtls,nmacs/tinydtls,fvdnabee/tinydtls,thecodemaiden/tinydtls-fork,jvermillard/tinydtls,nmacs/tinydtls