pptp-linux: add musl build fix patch

Fixes:
http://autobuild.buildroot.net/results/56d/56dd530d53489220d0080480310b8dc150cf1b2e/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>