gnss: truncate temporary file before download

A temporary file is used to download the GNSS assistance data.
It's created and truncated at initialization, but not truncated
when a re-upload is necessary (when data expires).
This causes to corrupt the GNSS assistance data if the new downloaded
data is smaller than the previous download.

src/gnss.c

@@ -191,6 +191,8 @@ static void fetch_assistance_data(struct EG25Manager *manager)
 
     /* Fetch assistance data with curl */
     tmp_file = fdopen(manager->gnss_assistance_fd, "wb+");
+    lseek(manager->gnss_assistance_fd, 0, SEEK_SET);
+    ftruncate(manager->gnss_assistance_fd, 0);
     url = g_strconcat(manager->gnss_assistance_url, "/",
                       manager->gnss_assistance_file, NULL);
     curl = curl_easy_init();

src/gnss.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <time.h>
+#include <unistd.h>
 #include <curl/curl.h>
 
 #include "manager.h"