Move out tbf subclasses from tbf.h to their own headers

It's a good start towards clearing current mess between parent and the 2
children classes.

Change-Id: Ibc22ea2e02609af7ee058b8bc15df2115d4c6f60
diff --git a/src/tbf.cpp b/src/tbf.cpp
index 37435f5..3ca39bf 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -21,6 +21,8 @@
 
 #include <bts.h>
 #include <tbf.h>
+#include <tbf_dl.h>
+#include <tbf_ul.h>
 #include <rlc.h>
 #include <encoding.h>
 #include <gprs_rlcmac.h>