libbpf: Fix build warning on ref_ctr_off for 32-bit architectures

Clang warns on 32-bit ARM on this comparision:

libbpf.c:10497:18: error: result of comparison of constant 4294967296 with expression of type 'size_t' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        if (ref_ctr_off >= (1ULL << PERF_UPROBE_REF_CTR_OFFSET_BITS))
            ~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Typecast ref_ctr_off to __u64 in the check conditional, it is false on
32bit anyways.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221219191526.296264-1-raj.khem@gmail.com