slabratetop: Add memcg_cache_params struct def

struct memcg_cache_params moved from include/linux/slab.h to
mm/slab.h in kernel v5.4, causing a compiler error when including
slub_def.h or slab_def.h in slabratetop's bpf program.

It has been removed completely from kernel version 5.9.

Add an empty memcg_cache_params struct in slabratetop's bpf
program so it will compile with kernel versions 5.4 to 5.8.

tools/slabratetop.py

@@ -62,7 +62,13 @@ def signal_ignore(signal, frame):
 bpf_text = """
 #include <uapi/linux/ptrace.h>
 #include <linux/mm.h>
-#include <linux/slab.h>
+
+// memcg_cache_params is a part of kmem_cache, but is not publicly exposed in
+// kernel versions 5.4 to 5.8. Define an empty struct for it here to allow the
+// bpf program to compile. It has been completely removed in kernel version
+// 5.9, but it does not hurt to have it here for versions 5.4 to 5.8.
+struct memcg_cache_params {};
+
 #ifdef CONFIG_SLUB
 #include <linux/slub_def.h>
 #else