netsnmp: depend on openssl when the package is on to ensure build order

The netsnmp package should depend on openssl when using it.
Otherwise netsnmp might get built before openssl and poison the
configure cache since it's not a mandatory dependency.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>