ab_tree: reduce visibility of "vs_empty" symbol

Fix one ASAN report about ODR violation

==112305==ERROR: AddressSanitizer: odr-violation (0x7fc5097a6160):
  [1] size=24 'vs_empty' lib/ab_tree.cpp:157:32 in /root/gromox/.libs/libgxh_ews.so.0
  [2] size=24 'vs_empty' lib/ab_tree.cpp:157:32 in /root/gromox/.libs/libgxp_exchange_nsp.so.0

Author: jengelh

include/gromox/ab_tree.hpp

@@ -256,8 +256,6 @@ class ab_base {
 	private:
 	const ab_domain *find_domain(uint32_t) const;
 
-	static const std::vector<std::string> vs_empty; ///< used to return empty alias list in case of invalid minid
-
 	GUID m_guid; ///< GUID of the base
 	gromox::time_point m_load_time{}; ///< Load time
 	/**

lib/ab_tree.cpp

@@ -154,7 +154,8 @@ void ab::work()
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 // ab_base organizational member functions
 
-const std::vector<std::string> ab_base::vs_empty{};
+/* Used to return empty alias list in case of invalid minid */
+static const std::vector<std::string> vs_empty;
 
 /**
  * @brief      Initialize base and lock until loaded