ibm: hypervisor-network-mgr: adjust includes and fwd declarations

clang-18 with C++23 complains as follows:

```
/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/unique_ptr.h:97:16: error: invalid application of 'sizeof' to an incomplete type 'phosphor::network::HypSysConfig'
   97 |         static_assert(sizeof(_Tp)>0,
...
../src/ibm/hypervisor-network-mgr-src/hyp_network_manager.hpp:159:32: note: in instantiation of member function 'std::unique_ptr<phosphor::network::HypSysConfig>::~unique_ptr' requested here
  159 |     SystemConfPtr systemConf = nullptr;
```

Get rid of the forward declaration and adjust some of the include
directives to satisfy clang.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Id2ebe1f5f7fce4962c0e3ffdf04bcaa476772e9a
5 files changed