Add fallback #DEFINE SRTP_SRCTP_INDEX_LEN

https://webrtc.googlesource.com/src/+/7738bc23ed7fee0d4856bdfe7b88985865829441
switched from using sizeof(uint32_t) to SRTP_SRCTP_INDEX_LEN.
It turned out that this is not always defined.
This patch defines it to 4.

BUG=webrtc:42222036

Change-Id: Ice3d24a6300d19bc2f573469aadd6474ace1b147
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/371220
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43548}
diff --git a/pc/srtp_session.cc b/pc/srtp_session.cc
index 509c2c9..bf27e63 100644
--- a/pc/srtp_session.cc
+++ b/pc/srtp_session.cc
@@ -34,6 +34,10 @@
 #include "third_party/libsrtp/include/srtp.h"
 #include "third_party/libsrtp/include/srtp_priv.h"
 
+#ifndef SRTP_SRCTP_INDEX_LEN
+#define SRTP_SRCTP_INDEX_LEN 4
+#endif
+
 namespace cricket {
 
 namespace {