Fix warning re: unread variable

[h3xx]

Fixes the following warning:

cc -Wall -O3 -D_GNU_SOURCE   -c -o mtftar.o mtftar.c
mtftar.c: In function 'main':
mtftar.c:115:13: warning: variable 'skipping_set' set but not used [-Wunused-but-set-variable]
  115 |         int skipping_set;
      |             ^~~~~~~~~~~~
cc   mtftar.o mtfscan.o mtfheader.o mtfstream.o tarout.o util.o md5/md5.o /usr/lib64/libm.so   -o mtftar

[h3xx]

Ready to go.