kcov: do not instrument lib/stackdepot.c

kcov: do not instrument lib/stackdepot.c

There's no point in collecting coverage from lib/stackdepot.c, as it is
not a function of syscall inputs.  Disabling kcov instrumentation for that
file will reduce the coverage noise level.

Link: http://lkml.kernel.org/r/1474640972-104131-1-git-send-email-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Kostya Serebryany <kcc@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>