x86: mkpiggy.c: Explicitly close the output file

Even though the resource is released when the application is closed or
when returned from main function, modify the code to make it obvious,
and to keep static analysis tools from complaining.

Signed-off-by: Geyslan G. Bem <geyslan@gmail.com>
Link: http://lkml.kernel.org/r/1381184219-10985-1-git-send-email-geyslan@gmail.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>