Find qplatformdefs.h for create-abi.

Authored by knauss on Dec 10 2018, 9:04 AM.

Description

Find qplatformdefs.h for create-abi.

The mkspecs directory is default Qt include directory. so we have to add
this. If we want to build ABIs for other devices, we would need to
modify this path accordingly.

Differential Revision: https://phabricator.kde.org/D17431

Details