windows: define variable only where used

windows: define variable only where used

warn_used is a static variable but that is used only under certain
conditions. These conditions do not apply for Windows and therefore
mingw is spitting a warning for unused variable.

Move variable declaration inside code block that uses it.

Fixes:

run_command.c: In function ‘openvpn_popen’:
run_command.c:236:17: warning: unused variable ‘warn_shown’
[-Wunused-variable]
  236 |     static bool warn_shown = false;
      |                 ^~~~~~~~~~

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20210501130640.9330-3-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22265.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>