scripts/mod/modpost.c: fix memory leak

sec2annotation returns malloc'ed buffer directly to printf as an argument.
 Free this buffer after printing.

Signed-off-by: Alexey Fomenko <ext-alexey.fomenko@nokia.com>
Cc: Trevor Keith <tsrk@tsrk.net>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>