Fix build of www/squid and www/squid-devel with clang 8

This fixes an error with a defaulted copy constructor:

../../src/security/ServerOptions.h:38:5: error: explicitly defaulted copy constructor is implicitly deleted [-Werror,-Wdefaulted-function-deleted]

ServerOptions(const ServerOptions &) = default;
^

../../src/security/ServerOptions.h:110:29: note: copy constructor of 'ServerOptions' is implicitly deleted because field 'clientCaStack' has a deleted copy constructor

X509_NAME_STACK_Pointer clientCaStack;
                        ^

/usr/include/c++/v1/memory:2494:3: note: copy constructor is implicitly deleted because 'unique_ptr<stack_st_X509_NAME, Security::ServerOptions::sk_X509_NAME_free_wrapper>' has a user-declared move constructor

unique_ptr(unique_ptr&& __u) noexcept
^

The copy constructor can instead be deleted.

Approved by: timp87@gmail.com (maintainer)
PR: 236210
MFH: 2019Q2