[partition] Fix missing initialization of the attribute partAttributes

- Initialize the attribute partAttributes to 0; it is a primitive type
  and it is not initialized in some constructors.
  Fixes commit c1b5426c6 ([partition] Add support for partition attributes).
- Move implementation of default constructor to cpp.

src/modules/partition/core/PartitionLayout.h

@@ -52,7 +52,7 @@ public:
         CalamaresUtils::Partition::PartitionSize partMaxSize;
 
         /// @brief All-zeroes PartitionEntry
-        PartitionEntry() {}
+        PartitionEntry();
         /// @brief Parse @p size, @p min and @p max to their respective member variables
         PartitionEntry( const QString& size, const QString& min, const QString& max );