lisa/_assets/kmodules/sched_tp/Makefile: Improve pahole output

Use --show_only_data_members to avoid having useless anonymous type
declaration in the middle of e.g. a struct.

Consider the following type:

    struct foo {
       struct { int a; } bar;
    };

GCC emits DWARF info looking this way:

    DW_TAG_structure_type
        DW_AT_name "foo"
        DW_TAG_member
            DW_AT_name "bar"
            DW_AT_type "struct "
            DW_AT_data_member_location <0x42 - curr_loc>
       NULL

    0x42:
    DW_TAG_structure_type
        DW_TAG_member
            DW_AT_name "a"
            DW_AT_type "int "
            DW_AT_data_member_location ...
        NULL

Clang emits:

    DW_TAG_structure_type
        DW_AT_name "foo"
        DW_TAG_member
            DW_AT_name "bar"
            DW_AT_type "struct "
            DW_AT_data_member_location 0x0

        DW_TAG_structure_type
            DW_TAG_member
                DW_AT_name "a"
                DW_AT_type "int "
                DW_AT_data_member_location ...
            NULL
        NULL

When pahole sees clang's output, it will emit something like that:

    struct foo {
       struct { int a; }; <------useless
       struct { int a; } bar;
    };

But with --show_only_data_members, it will ignore any child of
DW_TAG_structure_type that is not a DW_TAG_member and will print the
expected code.