lib/string_helpers.c: make arrays static

Moving these arrays into static storage shrinks the kernel a bit:

   text    data     bss     dec     hex filename
    723     112      64     899     383 lib/string_helpers.o
    516     272      64     852     354 lib/string_helpers.o

Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>