kstrtox: fix compile warnings in test

Fix the following warnings:

    CC [M]  lib/test-kstrtox.o
  lib/test-kstrtox.c: In function 'test_kstrtou64_ok':
  lib/test-kstrtox.c:318: warning: this decimal constant is unsigned only in ISO C90
	...

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>