folly: remove problematic -Wnon-virtual-dtor error from cmake build

Summary:
This causes problems with the cmake build because boost
triggers this warning, and we're promoting this to an error.

It is possible to use `-isystem` to tell the compiler that the
the boost headers are system headers, but introducing that for
boost headers is a mess.

We can safely live without this warning in the open source build
as our buck based builds have this enabled correctly.

Reviewed By: yfeldblum

Differential Revision: D14631786

fbshipit-source-id: cf30d0921ce592f410c2ad4bfddba1bddef01ed7

CMake/FollyCompilerUnix.cmake

@@ -42,7 +42,6 @@ function(apply_folly_compile_options_to_target THETARGET)
       -Wno-unused
       -Wunused-label
       -Wunused-result
-      -Wnon-virtual-dtor
       ${FOLLY_CXX_FLAGS}
   )
 endfunction()