gprs_gmm: Convert warning message to pragma message

This way we can enable -Werror in builds to disallow new compilation
warnings being introduced.

Change-Id: If2b9a618f4219df9e013095beff52c20aacbc79e
diff --git a/src/gprs/gprs_gmm.c b/src/gprs/gprs_gmm.c
index f4591f7..bc01681 100644
--- a/src/gprs/gprs_gmm.c
+++ b/src/gprs/gprs_gmm.c
@@ -2808,7 +2808,7 @@
 	switch (pdisc) {
 	case GSM48_PDISC_MM_GPRS:
 		rc = gsm0408_rcv_gmm(mmctx, msg, NULL, false);
-#warning "set drop_cipherable arg for gsm0408_rcv_gmm() from IuPS?"
+#pragma message "set drop_cipherable arg for gsm0408_rcv_gmm() from IuPS?"
 		break;
 	case GSM48_PDISC_SM_GPRS:
 		rc = gsm0408_rcv_gsm(mmctx, msg, NULL);