net/wmping: fix the port's build against -fno-common

Ensure that there is only one variable definition per each object, as
the C (and C++) standards mandated for years and compilers started to
enforce as of recently (Clang 11, GCC 10).

While here, define LICENSE, add missing USE_XORG components, and fix
-Wformat-security warnings, adding newline to printed error messages
for clarity and better look.

PR: 267587