colibri-vf: Fix EGLNative* typedefs to have a signed type

Chromium expects a signed type, and all other platforms also define
a signed type, like Display*, HDC, int, ANativeWindow*, void*.
This is a cherry-picked fix from qtglesstream to fix the QtWebEngine
build for colibri-vf.

Change-Id: I01d5a65b2e7512ebdc383501eacae1349a271039
Reviewed-by: Samuli Piippo <samuli.piippo@theqtcompany.com>