Explicit instanciation of Extendable<simgrid::s4u::Host>
It seems the `deleters_` static variable (of a template class) is not
uniquified on some systems (where the symbols are `STB_GLOBAL`) when
using Java loadLibrary leading to the JAVA_HOST_LEVEL being the same
as another level.
On some systems, the symbols are `STB_GNU_UNIQUE` which purpose is to
avoid this kind of behaviour and the bug does not happen.
By using explicit instanciation, the `deleters_` variable is not
allocated in each shared-object but only in `libsimgrid.so`.
