Declare Windows version of openvpn_execve() before use

Declare Windows version of openvpn_execve() before use

commit bf97c00f7dba441b504881f38e40afcbb610a39f moved
the generic openvpn_execve() to run_command.c and made it static.
But the Windows version is still in win32.c and is called from
run_command.c

Fix by declaring the function in win32.h

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1540436716-5725-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17825.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>