Moving compile_assert_c.h to webrtc/base

This header was not tracked by a GN target and in the discussion on
https://bugs.chromium.org/p/webrtc/issues/detail?id=7617 we decided
to also move it under webrtc/base.

I checked in chromium code search and it seems safe to move it
without creating a stub header in webrtc/system_wrappers.

BUG=webrtc:7617

Review-Url: https://codereview.webrtc.org/2882673002
Cr-Commit-Position: refs/heads/master@{#18151}

webrtc/modules/audio_coding/codecs/isac/main/source/pitch_filter.c

@@ -16,7 +16,7 @@
 
 #include "os_specific_inline.h"
 
-#include "webrtc/system_wrappers/include/compile_assert_c.h"
+#include "webrtc/base/compile_assert_c.h"
 
 /*
  * We are implementing the following filters;