cmake: make warning about headers not fatal

Warning issued on older boost and/or OS:

In file included from /usr/include/boost/asio/detail/socket_types.hpp:61:0,
                 from /usr/include/boost/asio/detail/epoll_reactor.hpp:30,
                 from /usr/include/boost/asio/detail/reactor.hpp:21,
                 from /usr/include/boost/asio/detail/impl/task_io_service.ipp:24,
                 from /usr/include/boost/asio/detail/task_io_service.hpp:198,
                 from /usr/include/boost/asio/impl/io_service.hpp:71,
                 from /usr/include/boost/asio/io_service.hpp:767,
                 from /usr/include/boost/asio/basic_io_object.hpp:19,
                 from /usr/include/boost/asio/basic_socket.hpp:20,
                 from /usr/include/boost/asio/basic_datagram_socket.hpp:20,
                 from /usr/include/boost/asio.hpp:21,
                 from /home/vagrant/slave/monero-static-alpine-3_5-x86_64/build/src/common/download.cpp:32:
/usr/include/sys/poll.h:1:2: warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
 #warning redirecting incorrect #include <sys/poll.h> to <poll.h>

CMakeLists.txt

@@ -407,7 +407,7 @@ else()
       set(WARNINGS "${WARNINGS} -Wno-error=inline-asm")
     endif()
   else()
-    set(WARNINGS "${WARNINGS} -Wlogical-op -Wno-error=maybe-uninitialized")
+    set(WARNINGS "${WARNINGS} -Wlogical-op -Wno-error=maybe-uninitialized -Wno-error=cpp")
   endif()
   if(MINGW)
     set(WARNINGS "${WARNINGS} -Wno-error=unused-value -Wno-error=unused-but-set-variable")