clang: Add microsoft extensions build flags

Starting in Linux 6.19, `struct ns_common` fields were moved into an
anonymous nested forward declaration (`struct ns_tree;`) within a
union.[1]
This causes Clang to throw a `[-Wmissing-declarations]` warning and
fail with missing member errors.

This affects tools pulling in `<linux/pid_namespace.h>` directly or
transitively (e.g. filetop, runqlat)

Errors encountered:
include/linux/ns/ns_common_types.h:117:3: warning: declaration does not declare anything [-Wmissing-declarations]
  117 |                 struct ns_tree;
      |                 ^~~~~~~~~~~~~~
In file included from /virtual/main.c:5:
In file included from include/linux/pid_namespace.h:11:
include/linux/ns_common.h:25:23: error: no member named 'ns_id' in 'struct ns_common'
   25 |         VFS_WARN_ON_ONCE(ns->ns_id == 0);
      |                          ~~  ^
include/linux/ns_common.h:26:13: error: no member named 'ns_id' in 'struct ns_common'
   26 |         return ns->ns_id <= NS_LAST_INIT_ID;
      |                ~~  ^
include/linux/ns_common.h:60:26: error: no member named '__ns_ref_active' in 'struct ns_common'
   60 |         return atomic_read(&ns->__ns_ref_active);

This patch fixes the issue globally by turning on microsoft extensions
flags. These flags have already been in the linux kernel. [2]

[1] torvalds/linux@a657bc8
[2] torvalds/linux@c4781dc

Signed-off-by: Avinesh Kumar <avinesh.kumar@suse.com>
Fixes: iovisor#5488

Author: Avinesh

src/cc/frontends/clang/kbuild_helper.cc

@@ -137,6 +137,10 @@ int KBuildHelper::get_flags(const char *uname_machine, vector<string> *cflags) {
   cflags->push_back("-Wno-pointer-sign");
   cflags->push_back("-fno-stack-protector");
 
+  // Align with Linux kernel's -fms-extensions flags adopted since v6.19
+  cflags->push_back("-fms-extensions");
+  cflags->push_back("-Wno-microsoft-anon-tag");
+
   /*
    * kernel is usually build with gcc and the kernel devel header
    * reflects that fact. However we build with clang and this must be