Move file_wrapper.h to rtc_base/system/

This moves it from an API directory (system_wrappers/include/) to a
non-API directory, which is exactly what we want for utilities like
this.

BUG=webrtc:8445

Change-Id: I440974da4d347b09ff042478720d7983056b62b9
Reviewed-on: https://webrtc-review.googlesource.com/21226
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22579}
diff --git a/modules/audio_processing/audio_processing_impl.cc b/modules/audio_processing/audio_processing_impl.cc
index 554dead..27388ea 100644
--- a/modules/audio_processing/audio_processing_impl.cc
+++ b/modules/audio_processing/audio_processing_impl.cc
@@ -45,7 +45,7 @@
 #include "modules/audio_processing/voice_detection_impl.h"
 #include "modules/include/module_common_types.h"
 #include "rtc_base/atomicops.h"
-#include "system_wrappers/include/file_wrapper.h"
+#include "rtc_base/system/file_wrapper.h"
 #include "system_wrappers/include/metrics.h"
 
 // Check to verify that the define for the intelligibility enhancer is properly