trace.hh: Fix assigner_type forward declaration

Spotted by Clang:

../../include/osv/trace.hh:58:1: error: 'assigner_type' defined as a struct template here but
      previously declared as a class template [-Werror,-Wmismatched-tags]
struct assigner_type<storage_args<s_args...>, runtime_args<r_args...>> {
^
../../include/osv/trace.hh:45:1: note: did you mean struct here?
class assigner_type;
^~~~~
struct

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
arch-cpu.hh: Fix arch_cpu forward declaration

Spotted by Clang:

../../include/sched.hh:278:12: error: class 'arch_cpu' was previously declared as a struct
      [-Werror,-Wmismatched-tags]
    friend class arch_cpu;
           ^
../../arch/x64/arch-cpu.hh:39:8: note: previous use is here
struct arch_cpu {
       ^
../../include/sched.hh:278:12: note: did you mean struct here?
    friend class arch_cpu;
           ^~~~~
           struct

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
sched.hh: Fix arch_cpu friend declaration

Spotted by Clang:

../../include/sched.hh:278:12: error: class 'arch_cpu' was previously
declared as a struct
      [-Werror,-Wmismatched-tags]
    friend class arch_cpu;
           ^
../../arch/x64/arch-cpu.hh:39:8: note: previous use is here
struct arch_cpu {
       ^
../../include/sched.hh:278:12: note: did you mean struct here?
    friend class arch_cpu;
           ^~~~~
           struct

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
sched.hh: Fix struct cpu friend and forward declarations

Spotted by Clang.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
elf.hh: Fix symbol_module forward declaration

Spotted by Clang.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
arch-cpu.hh: Fix arch_thread forward declaration

Spotted by Clang:

../../arch/x64/arch-cpu.hh:57:1: error: 'arch_thread' defined as a struct here but previously
      declared as a class [-Werror,-Wmismatched-tags]
struct arch_thread {
^
../../arch/x64/arch-cpu.hh:37:1: note: did you mean struct here?
class arch_thread;
^~~~~
struct

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>