serial.c: Use pragma message instead of warning

Change-Id: I65b1ef6e4b9ba0bf0c3c2f03311250f31ddb308a
diff --git a/src/serial.c b/src/serial.c
index dddad88..8ddd38e 100644
--- a/src/serial.c
+++ b/src/serial.c
@@ -202,7 +202,7 @@
 	}
 	return 0;
 #else
-#warning osmo_serial_set_custom_baudrate: unsupported platform
+#pragma message ("osmo_serial_set_custom_baudrate: unsupported platform")
 	return 0;
 #endif
 }