libbpf-tools: readahead: don't mark struct hist as static

libbpf-tools: readahead: don't mark struct hist as static

Libbpf readahead tool does not compile with bpftool v5.14. Since
commit 31332ccb756 ("bpftool: Stop emitting static variables in BPF
skeleton"), bpftool gen skeleton does not include static variables
into the skeleton file anymore.

Fixes the following compilation error:
readahead.c: In function 'main':
readahead.c:153:26: error: 'struct readahead_bpf__bss' has no member named 'hist'
  153 |         histp = &obj->bss->hist;
      |                          ^~