fixed missing variable for command line parsing

[pusi77]

Hey, i tried to install your latest release but i got this error:

arping.c: In function ‘arping_main’:
arping.c:2052:24: error: ‘c’ undeclared (first use in this function); did you mean ‘cp’?
 2052 |         while (EOF != (c = getopt(argc, argv,
      |                        ^
      |                        cp

I've found out that in one of your commits you removed the declaration of the "c" variable, i just re-added it :)

[ThomasHabets]

Ugh, that's embarrassing. Thanks!