Add -lstdc++ to Libs.Private to avoid the following static build failure
with nginx:
/home/buildroot/autobuild/instance-2/output-1/host/lib/gcc/xtensa-buildroot-linux-uclibc/10.3.0/../../../../xtensa-buildroot-linux-uclibc/bin/ld: /home/buildroot/autobuild/instance-2/output-1/host/bin/../xtensa-buildroot-linux-uclibc/sysroot/usr/lib/libmodsecurity.a(libmodsecurity_la-transaction.o): in function `std::basic_streambuf<char, std::char_traits<char> >::sbumpc() [clone .isra.0]':
transaction.cc:(.text+0x40): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_dispose()'