ntp: does not work with libressl

Fixes #10556

The --with-crypto handling in ntp only works with libopenssl, not with
libressl, where it ends up with compilation issues like:

ntp_control.c:(.text+0x64): undefined reference to `EVP_MD_CTX_new'
ntp_control.c:(.text+0x10c): undefined reference to `EVP_MD_CTX_free'
libntpd.a(ntp_crypto.o): In function `bighash':
ntp_crypto.c:(.text+0x2e8): undefined reference to `EVP_MD_CTX_new'
ntp_crypto.c:(.text+0x328): undefined reference to `EVP_MD_CTX_free'
libntpd.a(ntp_crypto.o): In function `crypto_verify':
ntp_crypto.c:(.text+0x6cc): undefined reference to `EVP_MD_CTX_new'
ntp_crypto.c:(.text+0x710): undefined reference to `EVP_MD_CTX_free'
ntp_crypto.c:(.text+0x72c): undefined reference to `EVP_MD_CTX_free'

So ensure we only pass --with-crypto when libopenssl is used.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>