imgdiff: fix file descriptor leak

mkstemp() allocates a file description that is never released.  If
MakePatch() is called too many time, imgdiff reaches the Operating
System EMFILE (too many open files) limit.

Change-Id: Icbe1399f6f6d32cfa1830f879cacf7d75bbd9fc3
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Signed-off-by: Gaelle Nassiet <gaellex.nassiet@intel.com>