RT6: Declare the struct tok srh_tlv_type "static"

RT6: Declare the struct tok srh_tlv_type "static"

This should avoid error like:
./print-rt6.c:36:18: error: no previous extern declaration for
non-static variable 'srh_tlv_type' [-Werror,-Wmissing-variable-declarations]
const struct tok srh_tlv_type[] = {
                 ^
./print-rt6.c:36:7: note: declare 'static' if the variable is not
intended to be used outside of this translation unit
const struct tok srh_tlv_type[] = {
      ^