Make libmariadb-dev depend on libssl-dev (Closes: #917135)

Anything that builds against libmariadb-dev expects to have libssl-dev
also available because libmariadb3 has these compilation flags:

pkg-config --libs libmariadb
-L/usr/lib/x86_64-linux-gnu/ -lmariadb -lz -ldl -lm -lpthread -lssl -lcrypto