package/opencv4: do not detect ccache

OpenCV-4's buildsystem will try to detect ccache and use it if
available. This may yield a system-installed ccache.

However, in Buildroot, ccache is entirely hidden away and handled in the
toolchain wrapper.

Forcibly disable detection of ccache.

Commit 505e7f47 already applied this
change to opencv3.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>