pgo: rectify comment to proper kernel-doc syntax

pgo: rectify comment to proper kernel-doc syntax

The command ./scripts/kernel-doc -none kernel/pgo/pgo.h warns:

  kernel/pgo/pgo.h:112: warning: cannot understand function prototype: 'struct llvm_prf_value_node_data '

This is due to a slightly invalid use of kernel-doc syntax for the comment
of this struct, that must have probably just slipped through refactoring
and review before.

Rectify the comment to proper kernel-doc syntax.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210628055947.6948-1-lukas.bulwahn@gmail.com