qtimageformats: Make the dependencies deterministic

* jasper and libwebp were autodetected from sysroot

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Revert "qtimageformats: Make the dependencies deterministic"

This reverts commit 088399a68731c138c03cc6857d6409b82b06ecb5.
qtwebkit.inc: add PACKAGECONFIG for libwebp

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>