package/jpeg: properly declare providers

package/jpeg: properly declare providers

libjpeg and jpeg-turo are providers of the jpeg virtual package, so they
must declare themselves as such.

We do not have any problem, because the dependency is computed from the
Kconfig-defined option BR2_PACKAGE_PROVIDES_JPEG, and we do not check
the Makefile-defined <FOO>_PROVIDES variable.

For the sake of correctness, make both libjpeg and jpeg-turo declare
themselves as providers for the jpeg virtual package.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>