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.h

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