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 3c202dd..0308b1b 100644
--- a/openbsc/tests/sgsn/Makefile.am
+++ b/openbsc/tests/sgsn/Makefile.am
@@ -34,5 +34,6 @@
 	$(LIBOSMOGSM_LIBS) \
 	$(LIBOSMOGB_LIBS) \
 	$(LIBCARES_LIBS) \
+	$(LIBCRYPTO_LIBS) \
 	-lgtp -lrt