doc: QtPlayer: check _WIN32 for windows plugins

QWindowsIntegrationPlugin is supposed to be linked against only when the
example is being compiled for Windows platform.

Cherry-picked from commit ce4e4085.

Signed-off-by: Alexandre Janniaux <alexandre.janniaux@gmail.com>

doc/libvlc/QtPlayer/main.cpp

@@ -14,7 +14,9 @@
 
 #ifdef QT_STATIC
 # include <QtPlugin>
+# ifdef _WIN32
 Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin)
+# endif
 #endif
 
 int main(int argc, char *argv[]) {