Made the pid number more readable

Changed the name from source to pid, to make it easier to read/understand.

Change-Id: I5ab70dab5694b3bff72ee8b120c35532df34cbdd
diff --git a/aos/logging/log_message.fbs b/aos/logging/log_message.fbs
index 789724f..7e246cf 100644
--- a/aos/logging/log_message.fbs
+++ b/aos/logging/log_message.fbs
@@ -18,7 +18,7 @@
   level:Level (id: 1);
 
   // Pid of the process creating the log message
-  source:int (id:2);
+  source_pid:int (id:2);
 
   // Application name
   name:string (id:3);