Fixed a compile problem with the GNU c++ library

modules/juce_core/memory/juce_ReferenceCountedObject.h

@@ -248,7 +248,7 @@ public:
 
    #if JUCE_COMPILER_SUPPORTS_NULLPTR
     /** Creates a pointer to a null object. */
-    ReferenceCountedObjectPtr (std::nullptr_t) noexcept
+    ReferenceCountedObjectPtr (decltype (nullptr)) noexcept
         : referencedObject (nullptr)
     {
     }

modules/juce_core/memory/juce_ScopedPointer.h

@@ -78,7 +78,7 @@ public:
 
    #if JUCE_COMPILER_SUPPORTS_NULLPTR
     /** Creates a ScopedPointer containing a null pointer. */
-    inline ScopedPointer (std::nullptr_t) noexcept   : object (nullptr)
+    inline ScopedPointer (decltype (nullptr)) noexcept   : object (nullptr)
     {
     }
    #endif