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}

common_audio/signal_processing/splitting_filter.c

@@ -44,9 +44,11 @@ static const uint16_t WebRtcSpl_kAllPassFilter2[3] = {21333, 49062, 63010};
 //                            |data_length|
 //
 
-void WebRtcSpl_AllPassQMF(int32_t* in_data, size_t data_length,
-                          int32_t* out_data, const uint16_t* filter_coefficients,
-                          int32_t* filter_state)
+static void WebRtcSpl_AllPassQMF(int32_t* in_data,
+                                 size_t data_length,
+                                 int32_t* out_data,
+                                 const uint16_t* filter_coefficients,
+                                 int32_t* filter_state)
 {
     // The procedure is to filter the input with three first order all pass filters
     // (cascade operations).