Fix the logic to suppress all warnings for iOS CI build. [ci skip]

Source/CMake/Modules/Urho3D-CMake-common.cmake

@@ -216,7 +216,7 @@ else ()
         endif ()
     elseif (IOS)
         # When performing CI build, suppress all the warnings for iOS build because 3rd party libraries produce too many of them and yet we don't want to touch the 3rd party library's source codes unnecessarily
-        if ($ENV{CI})
+        if ("$ENV{CI}" STREQUAL true)
             set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -w")
             set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -w")
         endif ()