src: gnss: fix build on 32-bit architectures

It seems `off_t` is now 64-bits wide on 32-bit architectures, so using
`glong` causes a build failure in this case. Switch to using the correct
type from the `sendfile()` prototype to fix this.

src/gnss.c

@@ -327,7 +327,7 @@ static void init_assistance_data_upload(struct EG25Manager *manager)
 static void upload_assistance_data(struct EG25Manager *manager)
 {
     gint error;
-    glong written_total = 0;
+    off_t written_total = 0;
     gint ret;
     struct stat sb;