Use the correct identifier for the OS X operating system.

This is consistent with other areas of Qt:
qmake, Qt.platform.os (QML), qbs.targetOS (QBS), and #define Q_OS_OSX.

Change-Id: Ibf98e6ba3556a14187dadae1b0235e9c907c9001
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/corelib/io/qfileselector.cpp

@@ -140,7 +140,7 @@ QFileSelectorPrivate::QFileSelectorPrivate()
 
     Selectors normally available are
     \list
-    \li platform, one of: android, blackberry, ios, windows, mac, linux, generic_unix or unknown.
+    \li platform, one of: android, blackberry, ios, windows, osx, linux, generic_unix or unknown.
     \li locale, same as QLocale::system().name().
     \endlist
 
@@ -360,8 +360,8 @@ QStringList QFileSelectorPrivate::platformSelectors()
     ret << QStringLiteral("windows");
 #elif defined(Q_OS_LINUX)
     ret << QStringLiteral("linux");
-#elif defined(Q_OS_MAC)
-    ret << QStringLiteral("mac");
+#elif defined(Q_OS_OSX)
+    ret << QStringLiteral("osx");
 #elif defined(Q_OS_UNIX)
     ret << QStringLiteral("generic_unix");
 #endif

tests/auto/corelib/io/qfileselector/qfileselector.qrc

@@ -14,7 +14,7 @@
    <file>platforms/+android/test</file>
    <file>platforms/+blackberry/test</file>
    <file>platforms/+ios/test</file>
-   <file>platforms/+mac/test</file>
+   <file>platforms/+osx/test</file>
    <file>platforms/+wince/test</file>
    <file>platforms/+windows/test</file>
    <file>platforms/+linux/test</file>