selftests/bpf: Ensure test flavors use correct skeletons

Ensure that test runner flavors include their own skeletons from <flavor>/
directory. Previously, skeletons generated for no-flavor test_progs were used.
Apart from fixing correctness, this also makes it possible to compile only
flavors individually:

  $ make clean && make test_progs-no_alu32
  ... now succeeds ...

Fixes: 74b5a596 ("selftests/bpf: Replace test_progs and test_maps w/ general rule")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200429012111.277390-2-andriin@fb.com

tools/testing/selftests/bpf/Makefile

@@ -324,7 +324,7 @@ $(TRUNNER_TEST_OBJS): $(TRUNNER_OUTPUT)/%.test.o:			\
 		      $(TRUNNER_BPF_SKELS)				\
 		      $$(BPFOBJ) | $(TRUNNER_OUTPUT)
 	$$(call msg,TEST-OBJ,$(TRUNNER_BINARY),$$@)
-	cd $$(@D) && $$(CC) $$(CFLAGS) -c $(CURDIR)/$$< $$(LDLIBS) -o $$(@F)
+	cd $$(@D) && $$(CC) -I. $$(CFLAGS) -c $(CURDIR)/$$< $$(LDLIBS) -o $$(@F)
 
 $(TRUNNER_EXTRA_OBJS): $(TRUNNER_OUTPUT)/%.o:				\
 		       %.c						\