mk: fix shared lib build with stable abi

mk: fix shared lib build with stable abi

When next ABI is enabled, the shared lib extension is .so.x.1.
That's why a double basename was introduced.
But the "ifeq NEXT_ABI" was forgotten, removing the .so
extension when NEXT_ABI is disabled.
It was preventing the linker from finding the .so libraries.

Fixes: 506f51cc0da7 ("mk: enable next abi preview")

Reported-by: John McNamara <john.mcnamara@intel.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: John McNamara <john.mcnamara@intel.com>