objtool: Fix whitelist documentation typo

The directive specified in the documentation to add an exception
for a single file in a Makefile was inverted.
Signed-off-by: Raphael Gault <raphael.gault@arm.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/522362a1b934ee39d0af0abb231f68e160ecf1a8.1557874043.git.jpoimboe@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/Documentation/stack-validation.txt

@@ -306,7 +306,7 @@ ignore it:
 
 - To skip validation of a file, add
 
-    OBJECT_FILES_NON_STANDARD_filename.o := n
+    OBJECT_FILES_NON_STANDARD_filename.o := y
 
   to the Makefile.