src/logging: no need to include log_protocol.h in log_manager.h

src/logging: no need to include log_protocol.h in log_manager.h

The header log_manager.h doesn't use anything from log_protocol.h and
the only other place than logging using log_protocol.h is qemu_command.c
where we can include log_protocol.h directly to have enum value
VIR_LOG_MANAGER_PROTOCOL_DOMAIN_OPEN_LOG_FILE_TRUNCATE available.

Fixes race-condition compilation error with meson:

In file included from ../tests/qemuhotplugmock.c:21:
In file included from ../src/qemu/qemu_hotplug.h:25:
In file included from ../src/qemu/qemu_domain.h:42:
../src/logging/log_manager.h:25:10: fatal error: 'logging/log_protocol.h' file not found

         ^~~~~~~~~~~~~~~~~~~~~~~~

1 error generated.

Reported-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>