usr.sbin/lpr/lpc: build warning fixes

[aprieger_llnw.com]

Summary

usr.sbin/lpr/lpc (line printer control program) build warning fixes

[aprieger_llnw.com]

Changelog:

1. usr.sbin/lpr/lpc/lpc.c
   • Warning
     • passing 'char *[20]' to parameter of type 'const char **' discards qualifiers in nested pointer types [-Wincompatible-pointer-types-discards-qualifiers]
   • Fix:
     • Explicitly cast the variable "margv" to const char ** only for it's use as a parameter to suppress the error