Eliminate include of <ctype.h>.

This is not built if we have glibc (as we already have getopt() from
it), and it's not part of glibc, so....

Change-Id: Iae4ff944c78fc55a8cd2eaf523a3696c47b741d7
Reviewed-on: https://code.wireshark.org/review/4769
Reviewed-by: Guy Harris <guy@alum.mit.edu>