Mac: Fix compilation with -qtnamespace

Change-Id: I85559b6698103eef865464a355d9b00012dcbd5f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/corelib/kernel/qcore_mac_objc.mm

@@ -41,6 +41,8 @@
 
 #include <private/qcore_mac_p.h>
 
+QT_BEGIN_NAMESPACE
+
 NSString *QCFString::toNSString(const QString &string)
 {
     // The const cast below is safe: CfStringRef is immutable and so is NSString.
@@ -52,3 +54,5 @@ QString QCFString::toQString(const NSString *nsstr)
     return toQString(reinterpret_cast<CFStringRef>(nsstr));
 }
 
+QT_END_NAMESPACE
+