import gsm_04_08_gprs.[ch] from openbsc.git

The definitions in this header file (and associated strings in the
c file) are generic about the GPRS Layer3 signalling protocols, and
thus should be part of the library.
diff --git a/include/Makefile.am b/include/Makefile.am
index 14a1871..49d1ce7 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -80,6 +80,7 @@
                        osmocom/gsm/protocol/gsm_03_40.h \
                        osmocom/gsm/protocol/gsm_03_41.h \
                        osmocom/gsm/protocol/gsm_04_08.h \
+                       osmocom/gsm/protocol/gsm_04_08_gprs.h \
                        osmocom/gsm/protocol/gsm_04_11.h \
                        osmocom/gsm/protocol/gsm_04_12.h \
                        osmocom/gsm/protocol/gsm_04_80.h \