pptp-linux: fix parallel build

Add a patch to fix a parallel build issue caused by a header file
being generated too late.

Fixes:

http://autobuild.buildroot.org/results/2324974d4818b2657a7df4beee33fc4713d6116f/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>