dtls: add static in front of inline
dtls: add static in front of inline

These functions where exported by multiple *.o files, because multiple
files used this header.

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

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