Fix Windows postbuild error

QGCSetup.pri

@@ -199,7 +199,7 @@ WindowsBuild {
     # Copy platform plugins
     P_DIR = $$[QT_INSTALL_PLUGINS]
     PLUGINS_DIR_WIN = $$replace(P_DIR, "/", "\\")
-    QMAKE_POST_LINK += $$escape_expand(\\n) mkdir -p "$$DESTDIR_WIN\\platforms"
+    QMAKE_POST_LINK += $$escape_expand(\\n) if not exist \"$$DESTDIR_WIN\\platforms\" mkdir \"$$DESTDIR_WIN\\platforms\"
     QMAKE_POST_LINK += $$escape_expand(\\n) $$QMAKE_COPY     \"$$PLUGINS_DIR_WIN\\platforms\\qwindows$${DLL_QT_DEBUGCHAR}.dll\" \"$$DESTDIR_WIN\\platforms\\qwindows$${DLL_QT_DEBUGCHAR}.dll\"
     QMAKE_POST_LINK += $$escape_expand(\\n) $$QMAKE_COPY_DIR \"$$PLUGINS_DIR_WIN\\imageformats\" \"$$DESTDIR_WIN\\imageformats\"
     QMAKE_POST_LINK += $$escape_expand(\\n) $$QMAKE_COPY_DIR \"$$PLUGINS_DIR_WIN\\sqldrivers\" \"$$DESTDIR_WIN\\sqldrivers\"