Always compile VP9 source files.

Instead of optionally compile VP9 source files based on the value of
the GN argument 'rtc_libvpx_build_vp9', this CL uses the preprocessor
macro RTC_ENABLE_VP9 to decide if VP9 related code needs to be compiled
or not.

Bug: None
Change-Id: I5c1b69d7ec35e8446181d98c912277d0ae8fdba2
Reviewed-on: https://webrtc-review.googlesource.com/c/111063
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25685}
diff --git a/modules/video_coding/codecs/vp9/vp9_impl.h b/modules/video_coding/codecs/vp9/vp9_impl.h
index a968e4c..d26c53d 100644
--- a/modules/video_coding/codecs/vp9/vp9_impl.h
+++ b/modules/video_coding/codecs/vp9/vp9_impl.h
@@ -12,6 +12,8 @@
 #ifndef MODULES_VIDEO_CODING_CODECS_VP9_VP9_IMPL_H_
 #define MODULES_VIDEO_CODING_CODECS_VP9_VP9_IMPL_H_
 
+#ifdef RTC_ENABLE_VP9
+
 #include <map>
 #include <memory>
 #include <vector>
@@ -179,4 +181,6 @@
 };
 }  // namespace webrtc
 
+#endif  // RTC_ENABLE_VP9
+
 #endif  // MODULES_VIDEO_CODING_CODECS_VP9_VP9_IMPL_H_