selftests/bpf: Fix undefined UINT_MAX in veristat.c

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

veristat.c: In function 'max_verifier_log_size':
veristat.c:1135:36: error: 'UINT_MAX' undeclared (first use in this function)
1135 | const int SMALL_LOG_SIZE = UINT_MAX >> 8;
| ^~~~~~~~
veristat.c:24:1: note: 'UINT_MAX' is defined in header '<limits.h>'; did you forget to '#include <limits.h>'?
23 | #include <math.h>
+++ |+#include <limits.h>
24 |

Fixes: 1f7c33630724 ("selftests/bpf: Increase verifier log limit in veristat")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250116075036.3459898-1-tony.ambardar@gmail.com