package/ninja: always use host-python3

package/ninja: always use host-python3

The current logic selects Python3 for the host only if Python3 is
selected for the target, otherwise it selects Python2.

As Meson, the only package infrastructure using ninja, needs Python 3,
it is desirable to also depend on Python 3 on the host for the ninja
host package.

Otherwise, if no Python interpreter is selected for the target, both
Python 2 and Python 3 are build for the host, which is time consuming
without any benefit.

For example when building libmpdclient (and all its target and host
dependencies) the actual elapsed time for is reduced from around 286s
to 207s as reported by `time -p make clean all`.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>