AppConfig: Ensure AppConfig is always included in plugin wrappers

Previously, the `JUCE_APP_CONFIG_HEADER` macro wasn't being defined/included
in some of the wrapper files, which meant that there was a danger of these
files building without important JucePlugin_Build_* macros defined.

modules/juce_audio_plugin_client/AU/juce_AU_Wrapper.mm

@@ -22,6 +22,8 @@
 
   ==============================================================================
 */
+
+#include <juce_core/system/juce_TargetPlatform.h>
 #include <juce_core/system/juce_CompilerWarnings.h>
 #include "../utility/juce_CheckSettingMacros.h"
 

modules/juce_audio_plugin_client/Unity/juce_Unity_Wrapper.cpp

@@ -23,9 +23,10 @@
   ==============================================================================
 */
 
+#include <juce_core/system/juce_TargetPlatform.h>
+
 #if JucePlugin_Build_Unity
 
-#include <juce_core/system/juce_TargetPlatform.h>
 #include "../utility/juce_IncludeModuleHeaders.h"
 #include <juce_audio_processors/format_types/juce_LegacyAudioParameter.cpp>
 

modules/juce_audio_plugin_client/juce_audio_plugin_client_AU_2.mm

@@ -23,6 +23,8 @@
   ==============================================================================
 */
 
+#include <juce_core/system/juce_TargetPlatform.h>
+
 #if JucePlugin_Build_AU
 
 #include <juce_core/system/juce_CompilerWarnings.h>

modules/juce_audio_plugin_client/juce_audio_plugin_client_Standalone.cpp

@@ -23,6 +23,8 @@
   ==============================================================================
 */
 
+#include <juce_core/system/juce_TargetPlatform.h>
+
 #if JucePlugin_Build_Standalone
 
 #if ! JUCE_MODULE_AVAILABLE_juce_audio_utils