libbpf: remove compile time warning from libbpf_util.h

Having a helpful compile time warning in libbpf_util.h is not a good
idea since all warnings are treated as errors. Change this into a
comment in the code instead.

Fixes: b7e3a280 ("libbpf: remove dependency on barrier.h in xsk.h")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf_util.h

@@ -46,7 +46,7 @@ do {				\
 # define libbpf_smp_mb() asm volatile("dmb ish" : : : "memory")
 # define libbpf_smp_rwmb() libbpf_smp_mb()
 #else
-# warning Architecture missing native barrier functions in libbpf_util.h.
+/* Architecture missing native barrier functions. */
 # define libbpf_smp_rmb() __sync_synchronize()
 # define libbpf_smp_wmb() __sync_synchronize()
 # define libbpf_smp_mb() __sync_synchronize()