veristat: Fix a memory leak for preset ENUMERATOR

ASAN detected a memory leak in veristat. The cleanup code handling
ENUMERATOR value missed freeing strdup-ed svalue. Fix it.

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223190736.649171-13-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/veristat.c

@@ -3378,6 +3378,8 @@ int main(int argc, char **argv)
 			}
 		}
 		free(env.presets[i].atoms);
+		if (env.presets[i].value.type == ENUMERATOR)
+			free(env.presets[i].value.svalue);
 	}
 	free(env.presets);
 	return -err;