package/opencv3: do not detect ccache

OpenCV-3'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.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>