Build WindowsWeakSymbol.cpp only with unique globals enabled.

Otherwise this function is not used from any code. Same as the
corresponding change in Corrade.

src/Magnum/CMakeLists.txt

@@ -126,7 +126,7 @@ if(MAGNUM_BUILD_DEPRECATED)
 endif()
 
 # Functionality specific to static Windows builds
-if(CORRADE_TARGET_WINDOWS AND NOT CORRADE_TARGET_WINDOWS_RT AND MAGNUM_BUILD_STATIC)
+if(CORRADE_TARGET_WINDOWS AND NOT CORRADE_TARGET_WINDOWS_RT AND MAGNUM_BUILD_STATIC AND MAGNUM_BUILD_STATIC_UNIQUE_GLOBALS)
     list(APPEND Magnum_SRCS Implementation/WindowsWeakSymbol.cpp)
     list(APPEND Magnum_PRIVATE_HEADERS Implementation/WindowsWeakSymbol.h)
 endif()