util: Fix incorrect return type in m5 writefile

Change-Id: Ic24a1c3c1488e970ed27bb6b99262d201f535384
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2960
Reviewed-by: Gabe Black <gabeblack@google.com>

util/m5/m5.c

@@ -122,7 +122,7 @@ read_file(int dest_fid)
     }
 }
 
-int
+void
 write_file(const char *filename)
 {
     fprintf(stderr, "opening %s\n", filename);