package/mender: force libopenssl

mender raises the following build failure with libressl since at least
version 3.1.0 (and probably since the addition of the package):

vendor/github.com/mendersoftware/openssl/conn.go:103:61: could not determine kind of name for C.X509_V_ERR_DANE_NO_MATCH

Fixes:
 - http://autobuild.buildroot.org/results/2b032d2a4e7cfbb20444c13821003120a4d00abd
 - http://autobuild.buildroot.org/results/621c057c0b8b627c8140456a2b0af4f95d239dcc



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>