dcesrv_core: fix build

Move include of system/network.h above dcesrv_init_context to avoid build error:

In file included from ../../lib/replace/system/network.h:35,
                 from ../../librpc/rpc/dcesrv_core.c:2658:
usr/include/unistd.h: At top level:
usr/include/unistd.h:675:16: error: conflicting types for ‘geteuid’
  675 | extern __uid_t geteuid (void) __THROW;

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• CI timeout is 3h or higher (see Settings/CICD/General pipelines/ Timeout)