package/zstd: avoid compilation during host-zstd install step

package/zstd: avoid compilation during host-zstd install step

The host-zstd-build step was not actually compiling the library:

make[1]: Entering directory '/buildroot/output/build/host-zstd-1.4.5/lib'
make[1]: Nothing to be done for 'default'.
make[1]: Leaving directory '/buildroot/output/build/host-zstd-1.4.5/lib'

and the actual compilation was part of the install step.
This is not how other Buildroot packages work.

Make sure to specify which library targets we want instead. The total amount
of compiled files does not change with this patch.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>