pool: Use uintptr_t for pool thread magic number

This ensures that it's the same size as void *, so that casting to that
type won't trigger a diagnostic on any target. This fixes an armhf build
failure.

Signed-off-by: Cole Miller <[email protected]>

src/lib/threadpool.c

@@ -76,7 +76,7 @@ static const struct sm_conf planner_states[PS_NR] = {
 	},
 };
 
-static const uint64_t pool_thread_magic = 0xf344e2;
+static const uintptr_t pool_thread_magic = 0xf344e2;
 static uv_key_t thread_identifier_key;
 
 enum {