Add a GSM RLP decoder and encoder

This code implements a decoder and encoder for the RLP (Radio Link
Protocol) as used in the bearer channel of GSM CSD (Circuit Switched
Data).

Change-Id: I2d9bd8eb4f0cd0f72c436996767b199429596917
diff --git a/include/osmocom/gsm/Makefile.am b/include/osmocom/gsm/Makefile.am
index f840a27..5678a51 100644
--- a/include/osmocom/gsm/Makefile.am
+++ b/include/osmocom/gsm/Makefile.am
@@ -48,6 +48,7 @@
 	l1sap.h \
 	oap.h \
 	oap_client.h \
+	rlp.h \
 	rsl.h \
 	rxlev_stat.h \
 	sysinfo.h \