gprs: Use RAND_bytes for p-tmsi

[hfreyther: Link to libcrypto, include header, add uint8_t* cast]
diff --git a/openbsc/tests/sgsn/Makefile.am b/openbsc/tests/sgsn/Makefile.am
index 2782d9e..4c6ba9f 100644
--- a/openbsc/tests/sgsn/Makefile.am
+++ b/openbsc/tests/sgsn/Makefile.am
@@ -35,5 +35,6 @@
 	$(LIBOSMOGSM_LIBS) \
 	$(LIBOSMOGB_LIBS) \
 	$(LIBCARES_LIBS) \
+	$(LIBCRYPTO_LIBS) \
 	-lgtp -lrt