kernel: avoid potential duplicate object in shared library/binary

Fixes warning and potential bug whereby init_flag may exist in both
libbitcoinkernel as well as a downstream user, as opposed to being shared as
intended.

src/support/lockedpool.h:224:31:
warning: 'init_flag' may be duplicated when built into a shared library: it is mutable, has hidden visibility, and external linkage [-Wunique-object-duplication]

src/support/lockedpool.cpp

@@ -399,3 +399,10 @@ void LockedPoolManager::CreateInstance()
     static LockedPoolManager instance(std::move(allocator));
     LockedPoolManager::_instance = &instance;
 }
+
+LockedPoolManager& LockedPoolManager::Instance()
+{
+    static std::once_flag init_flag;
+    std::call_once(init_flag, LockedPoolManager::CreateInstance);
+    return *LockedPoolManager::_instance;
+}

src/support/lockedpool.h

@@ -219,12 +219,7 @@ class LockedPoolManager : public LockedPool
 {
 public:
     /** Return the current instance, or create it once */
-    static LockedPoolManager& Instance()
-    {
-        static std::once_flag init_flag;
-        std::call_once(init_flag, LockedPoolManager::CreateInstance);
-        return *LockedPoolManager::_instance;
-    }
+    static LockedPoolManager& Instance();
 
 private:
     explicit LockedPoolManager(std::unique_ptr<LockedPageAllocator> allocator);