Organize iLBC headers as per style guide

Prior to this commit, most .c files in modules/audio_coding/codecs/ilbc
don't include their corresponding headers, nor do they order #includes
as per the Google Style Guide [1]. The former is especially harmful,
since in C it can silently allow the function signature to diverge from
its prototype, thus causing disaster at runtime.

This CL fixes both issues. In effect, this allows the common_audio and
modules/audio_coding:ilbc targets to be compiled with Clang's
-Wmissing-prototypes, though this CL does not add that change.

[1]: https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes

Bug: webrtc:12314
Change-Id: I8299968ed3cc86ff35d9de045072b846298043af
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/198362
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Timothy Gu <timothygu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#32896}

modules/audio_coding/codecs/ilbc/encode.h

@@ -19,6 +19,9 @@
 #ifndef MODULES_AUDIO_CODING_CODECS_ILBC_MAIN_SOURCE_ENCODE_H_
 #define MODULES_AUDIO_CODING_CODECS_ILBC_MAIN_SOURCE_ENCODE_H_
 
+#include <stddef.h>
+#include <stdint.h>
+
 #include "modules/audio_coding/codecs/ilbc/defines.h"
 
 /*----------------------------------------------------------------*