src: Move the decision to build the ISA plugin to the top level make file

src: Move the decision to build the ISA plugin to the top level make file

Previously, the first time you build ceph, common did not see the correct
value of WITH_EC_ISA_PLUGIN.  The consequence is that the global.yaml gets
build with osd_erasure_code_plugins not including isa.  This is not great
given its our default plugin.

We considered simply removing this parameter from make entirely, but this
may require more discussion about supporting old hardware.

So the slightly ugly fix is to move this erasure-code specific declartion
to the top-level.

Fixes: https://tracker.ceph.com/issues/75537
Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>