opencv: always enable opencv_core module when opencv is enabled

It does not make much sense enabling opencv without its core module.

This configuration leads to build nothing (since all modules depend on
the core one), but install the configuration files (*.pc and *.cmake)
anyway.

This absurd situation may break the build-system of other packages
that would correctly find the *.pc (but does not check for the modules
they actually use), but would not build because of missing headers and
libraries.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>