Missing return wsi_alloc_utils.hpp in WSI layer

Ensure the move-assignment operator in
swapchain_wsialloc_allocator returns *this on all
code paths. This avoids undefined behavior and satisfies
the function’s return type requirements.

Signed-off-by: Maged Elnaggar <maged.elnaggar@arm.com>
Change-Id: I99762484d7a02c24a4632a7a84b415a3842d02a3

wsi/wsi_alloc_utils.hpp

@@ -75,6 +75,7 @@ public:
          m_allocator = nullptr;
       }
       std::swap(m_allocator, other.m_allocator);
+      return *this;
    }
 
    ~swapchain_wsialloc_allocator()