lib: pdn: explicitly request HEAP size

[JordanYates]

The PDN library requires the system heap, so it should be declaring a Kconfig symbol of type HEAP_MEM_POOL_ADD_SIZE_ to request what it needs.

The old dependency is incorrect since the system heap can still exist even when CONFIG_HEAP_MEM_POOL_SIZE == 0.

[NordicBuilder]

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