maint: avoid uniq.c warning from bleeding-edge gcc's -Wstrict-overflow

maint: avoid uniq.c warning from bleeding-edge gcc's -Wstrict-overflow

* src/uniq.c (main): Make the type of "nfiles" unsigned,
to avoid a brand new warning from a gcc I built from today's
sources (gcc version 6.0.0 20151015 (experimental) (GCC)):
src/uniq.c:523:14: error: assuming signed overflow does not occur \
  when simplifying conditional to constant [-Werror=strict-overflow]
           if (nfiles == 2)
              ^