qt5base: Fix INCLUDE_PATH when no JPEG/PNG support

Fixes:
  http://autobuild.buildroot.net/results/4be77432390c4cfb557c3f2cc52cf732de037949/
  http://autobuild.buildroot.net/results/255a77e01d8af4090e03acb82f9ad93b7c4dd212/
  http://autobuild.buildroot.net/results/7844e6a3eeecf67dc29277b73bfe714e4d8f0d85/
  http://autobuild.buildroot.net/results/b941f35b6c8e1cf3b2c4b046dfcce98700d8f408/
  http://autobuild.buildroot.net/results/c64b640a39285f59cde47cb8d06ab70a5ac97c8b/
  http://autobuild.buildroot.net/results/f6d24b50f58251c8051d39bc1d09b5b43f81f441/
  http://autobuild.buildroot.net/results/30acea45fb497942129ca57c143bab6174e14609/
  http://autobuild.buildroot.net/results/f0891474fe2cd3e47b1489f147e31f698dbd013f/

Upstream-Status: https://codereview.qt-project.org/#/c/122145/



Signed-off-by: Julien Corjon <corjon.j@ecagroup.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>