Bugfix: build error with DISTRHO_PLUGIN_HAS_EXTERNAL_UI (#323)

* Bugfix: build error with DISTRHO_PLUGIN_HAS_EXTERNAL_UI

Update DIstrhoUI.cpp to include X11 headers before DPF stuff to prevent
X11 Window and DGL:Window from clashing.

* External UI: avoid including a DGL header

distrho/src/DistrhoUI.cpp

@@ -14,8 +14,8 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#include "DistrhoUIPrivateData.hpp"
-#include "src/WindowPrivateData.hpp"
+#include "src/DistrhoPluginChecks.h"
+
 #if DISTRHO_PLUGIN_HAS_EXTERNAL_UI
 # if defined(DISTRHO_OS_WINDOWS)
 #  define WIN32_LEAN_AND_MEAN
@@ -25,8 +25,11 @@
 # endif
 #else
 # include "src/TopLevelWidgetPrivateData.hpp"
+# include "src/WindowPrivateData.hpp"
 #endif
 
+#include "DistrhoUIPrivateData.hpp"
+
 START_NAMESPACE_DISTRHO
 
 #if DISTRHO_PLUGIN_HAS_EXTERNAL_UI