objtool: Skip samples subdirectory

The code in the 'samples' subdirectory isn't part of the kernel, so
there's no need to validate it.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/c4cb4ef635ec606454ab834cb49fc3e9381fb1b1.1579543924.git.jpoimboe@redhat.com