selftests/bpf: Fix missing UINT_MAX definitions in benchmarks

Include <limits.h> in 'bench.h' to provide a UINT_MAX definition and avoid
multiple compile errors against mips64el/musl-libc like:

  benchs/bench_local_storage.c: In function 'parse_arg':
  benchs/bench_local_storage.c:40:38: error: 'UINT_MAX' undeclared (first use in this function)
     40 |                 if (ret < 1 || ret > UINT_MAX) {
        |                                      ^~~~~~~~
  benchs/bench_local_storage.c:11:1: note: 'UINT_MAX' is defined in header '<limits.h>'; did you forget to '#include <limits.h>'?
     10 | #include <test_btf.h>
    +++ |+#include <limits.h>
     11 |

seen with bench_local_storage.c, bench_local_storage_rcu_tasks_trace.c, and
bench_bpf_hashmap_lookup.c.

Fixes: 73087489 ("selftests/bpf: Add benchmark for local_storage get")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/8f64a9d9fcff40a7fca090a65a68a9b62a468e16.1721713597.git.tony.ambardar@gmail.com

tools/testing/selftests/bpf/bench.h

@@ -10,6 +10,7 @@
 #include <math.h>
 #include <time.h>
 #include <sys/syscall.h>
+#include <limits.h>
 
 struct cpu_set {
 	bool *cpus;