Fix builds on Linux CI

This fixes compilation on older Linux compilers (gcc 7.5.0, clang 7.0.0)
that do not seem to implement P0558R1 defect report, thus not allowing
the usage of value_type member typedef from std::atomic.

include/vk_mem_alloc.h

@@ -4006,6 +4006,8 @@ template<typename AtomicT>
 struct AtomicTransactionalIncrement
 {
 public:
+    using T = decltype(AtomicT().load());
+
     ~AtomicTransactionalIncrement()
     {
         if(m_Atomic)
@@ -4013,7 +4015,7 @@ public:
     }
 
     void Commit() { m_Atomic = nullptr; }
-    typename AtomicT::value_type Increment(AtomicT* atomic)
+    T Increment(AtomicT* atomic)
     {
         m_Atomic = atomic;
         return m_Atomic->fetch_add(1);