Bug 5319: QOS Netfilter MARK preservation is always disabled (#1585)

Bug 5319: QOS Netfilter MARK preservation is always disabled (#1585)

Default ./configure options and explicit --enable-zph-qos enabled ZPH
QOS support (USE_QOS_TOS) as expected but did not enable QOS Netfilter
MARK preservation support (USE_LIBNETFILTERCONNTRACK). For example,
qos_flows directive became available, but clientside_mark and
client_connection_mark ACL types were not recognized.

The missing opening bracket before AS_IF() condition injected a trailing
closing bracket into that condition, resulting in an always-false
condition for setting USE_LIBNETFILTERCONNTRACK.

Broken since 2022 commit a1c2236.