GCC12.2 warns about dereferencing a pointer to RpcPktHdr
while it has been allocated to the size of one of the packet
(hence smaller in some cases).

/home/eric/work/wine/dlls/rpcrt4/rpc_message.c:111:26: warning: array subscript 'RpcPktHdr[0]' is partly outside array bounds of 'unsigned char[24]' [-Warray-bounds]
  111 |   Header->common.rpc_ver = RPC_VER_MAJOR;

This patch fixes the warnings by accessing the created object
through a pointer to their type (and not through the union).
Signed-off-by: Eric Pouech <eric.pouech@gmail.com>