Fix USE_SYSTEM_OPENAL to use system headers

Even when USE_SYSTEM_OPENAL was enabled, the code directly included the
bundled OpenAL headers, bypassing the system headers.

Since `#include <AL/al.h>` is not portable, use `#include "al.h"`
instead.
https://cmake.org/cmake/help/latest/module/FindOpenAL.html

rpcs3/Emu/Cell/Modules/cellMic.cpp

@@ -11,7 +11,7 @@
 #include <cmath>
 
 #ifndef WITHOUT_OPENAL
-#include "3rdparty/OpenAL/openal-soft/include/AL/alext.h"
+#include "alext.h"
 #endif
 
 LOG_CHANNEL(cellMic);

rpcs3/Emu/Cell/Modules/cellMic.h

@@ -1,9 +1,10 @@
 #pragma once
 
 #include "Utilities/Thread.h"
-#include "3rdparty/OpenAL/openal-soft/include/AL/alc.h"
 #include "Utilities/mutex.h"
 
+#include "alc.h"
+
 // Error Codes
 enum CellMicInError : u32
 {

rpcs3/rpcs3qt/microphone_creator.cpp

@@ -3,8 +3,8 @@
 
 #include "Utilities/StrUtil.h"
 
-#include "3rdparty/OpenAL/openal-soft/include/AL/al.h"
-#include "3rdparty/OpenAL/openal-soft/include/AL/alc.h"
+#include "al.h"
+#include "alc.h"
 
 LOG_CHANNEL(cfg_log, "CFG");