core: fix -Wunused-but-set-variable

When configured with -DSeastar_NUMA=OFF, fix -Wunused-but-set-variable
that triggers with clang-16.

seastar/src/core/memory.cc:1868:12: error: variable 'pos' set but not used [-Werror,-Wunused-but-set-variable]
    size_t pos = 0;

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>

src/core/memory.cc

@@ -1865,7 +1865,7 @@ configure(std::vector<resource::memory> m, bool mbind,
         get_cpu_mem().replace_memory_backing(sys_alloc);
     }
     get_cpu_mem().resize(total, sys_alloc);
-    size_t pos = 0;
+    [[maybe_unused]] size_t pos = 0;
     for (auto&& x : m) {
 #ifdef SEASTAR_HAVE_NUMA
         unsigned long nodemask = 1UL << x.nodeid;