redhat: rh_flags: declare proper static methods when !CONFIG_RHEL_DIFFERENCES

rh_print_flags() and rh_add_flag() are defined as prototypes when CONFIG_RHEL_DIFFERENCES is not defined leading to the following CLANG woe:

./include/linux/rh_flags.h:32:1: note: declare 'static' if the function is not intended to be used outside of this translation unit 32 | void rh_add_flag(const char *flag_name) { };
| ^
| static
2 warnings generated.

breaking CKI_Rawhide_ClangLTO pipelines

Signed-off-by: Rafael Aquini aquini@redhat.com