https://github.com/iovisor/bcc/pull/5555

Prevent a warning caused by some typeof(x) expressions:

In file included from /virtual/main.c:4:
In file included from include/net/tcp.h:34:
In file included from include/net/xfrm.h:26:
include/net/ip6_fib.h:256:10: warning: default initialization of an object of type 'typeof (f6i->expires)' (aka 'const unsigned long')
      leaves the object uninitialized [-Wdefault-const-init-var-unsafe]
  256 |                 return time_after(jiffies, f6i->expires);
      |                        ^
include/linux/jiffies.h:75:3: note: expanded from macro 'time_after'
   75 |          typecheck(unsigned long, b) && \
      |          ^
include/linux/typecheck.h:8:12: note: expanded from macro 'typecheck'
    8 |         typeof(x) __dummy2; \
      |                   ^
1 warning generated.

Add -Wno-default-const-init-var-unsafe to silence this.

--- a/src/cc/frontends/clang/kbuild_helper.cc
+++ b/src/cc/frontends/clang/kbuild_helper.cc
@@ -141,6 +141,9 @@ int KBuildHelper::get_flags(const char *
   cflags->push_back("-fms-extensions");
   cflags->push_back("-Wno-microsoft-anon-tag");
 
+  // Prevent warnings from typeof(x) expressions in some kernel headers
+  cflags->push_back("-Wno-default-const-init-var-unsafe");
+
   /*
    * kernel is usually build with gcc and the kernel devel header
    * reflects that fact. However we build with clang and this must be