bcc/tools: update mountsnoop's based on comment in containers.py

this patch just replicates the fix done in
ef330a3

tools/mountsnoop.py

@@ -33,7 +33,10 @@
  * real struct, but we don't need them, and they're more likely to change.
  */
 struct mnt_namespace {
+ // This field was removed in https://github.com/torvalds/linux/commit/1a7b8969e664d6af328f00fe6eb7aabd61a71d13
+ #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 11, 0)
  atomic_t count;
+ #endif
  struct ns_common ns;
 };