Fixed diskfree() returning garbage value

Always include platform.h before system headers. Otherwise types such as
off_t will use 32-bit layout instead of the 64-bit layout required by
_FILE_OFFSET_BITS=64 on HP-UX.

Ticket: ENT-13531
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>
(cherry picked from commit c30bead)

Author: larsewi

libpromises/storage_tools.c

@@ -22,6 +22,8 @@
   included file COSL.txt.
 */
 
+#include <platform.h>
+
 #include <cf3.defs.h>
 
 #ifdef HAVE_SYS_STATFS_H