UBUNTU: SAUCE: selftests/bpf: ignore pointer types check with clang

Building bpf selftests with clang can trigger errors like the following:

  CLNG-BPF [test_maps] bpf_iter_netlink.bpf.o
progs/bpf_iter_netlink.c:32:4: error: incompatible pointer types assigning to 'struct sock *' from 'struct sock___17 *' [-Werror,-Wincompatible-pointer-types]
        s = &nlk->sk;
          ^ ~~~~~~~~
1 error generated.

This is due to the fact that bpftool emits duplicate data types with
different names in vmlinux.h (i.e., `struct sock` in this case) and
these types, despite having a different name, represent in fact the same
object.

Add -Wno-incompatible-pointer-types to CLANG_CLAGS to prevent these
errors.

(cherry picked from https://lore.kernel.org/lkml/20230412095912.188453-1-andrea.righi@canonical.com/T/#u)
[ arighi: drop this when we move to pahole >= 1.25 ]
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

tools/testing/selftests/bpf/Makefile

@@ -386,7 +386,8 @@ BPF_CFLAGS = -g -Wall -Werror -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN)	\
 # TODO: enable me -Wsign-compare
 
 CLANG_CFLAGS = $(CLANG_SYS_INCLUDES) \
-	       -Wno-compare-distinct-pointer-types
+	       -Wno-compare-distinct-pointer-types \
+	       -Wno-incompatible-pointer-types
 
 $(OUTPUT)/test_l4lb_noinline.o: BPF_CFLAGS += -fno-inline
 $(OUTPUT)/test_xdp_noinline.o: BPF_CFLAGS += -fno-inline