sparc: fix for missing include file

[Paul Eggert]

David Miller wrote:
> strncat.c: In function ‘strncat’:
> strncat.c:76:6: error: ‘c’ may be used uninitialized in this function [-Werror=uninitialized]
>
> This is the generic C strncat.c in string/strncat.c, and actually the
> compiler is right here in that when 'n' is zero 'c' will in fact be
> tested without being initialized to anything.

That's amusing, since the code works regardless of whether 'c' is initialized 
properly, assuming that using an uninitialized value doesn't trap or have 
similarly bad behavior.  So this looks like another candidate for adding pragmas 
to suppress the diagnostic.  In Gnulib we'd say 'char c IF_LINT (= 0);'.