diffutils: fix build with recent glibc

Fixes the following build problem with recent toolchains:

In file included from exclude.c:31:0:
./stdio.h:1012:1: error: 'gets' undeclared here (not in a function)
  CC       hard-locale.o
make[4]: *** [exclude.o] Error 1
make[4]: *** Waiting for unfinished jobs....
In file included from stdio-safer.h:20:0,
                 from freopen-safer.c:22:
./stdio.h:1012:1: error: 'gets' undeclared here (not in a function)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>