openldap: correct openssl handling

openldap: correct openssl handling

Fixes:
http://autobuild.buildroot.net/results/ffc/ffc9c10c55c2838ab7002c5ec35244e9bfe46189/

Commit dfa1817d31a (openldap: supports only the real OpenSSL, not LibreSSL)
tried to ensure openldap would only use openssl, but changed the wrong
variable. OPENLDAP_TLS is passed to configure, so it shouldn't be changed:

./configure --target=aarch64-buildroot-linux-gnu .. --with-tls=libopenssl ..
Configuring OpenLDAP 2.4.45-Release ...
checking build system type... x86_64-pc-linux-gnu
checking host system type... aarch64-buildroot-linux-gnu
checking target system type... aarch64-buildroot-linux-gnu
checking for a BSD-compatible install... /usr/bin/install -c
checking whether build environment is sane... yes
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking for aarch64-buildroot-linux-gnu-strip... /usr/lfs/v0/rc-buildroot-test/scripts/instance-0/output/host/bin/aarch64-linux-gnu-strip
checking configure arguments... configure: error: bad value libopenssl for --with-tls

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>