selftests: bpf: tests.h should depend on .c files, not the output

This makes sure we don't put headers as input files when doing
compilation, because clang complains about the following:

clang-9: error: cannot specify -o when generating multiple output files
../lib.mk:152: recipe for target 'xxx/tools/testing/selftests/bpf/test_verifier' failed
make: *** [xxx/tools/testing/selftests/bpf/test_verifier] Error 1
make: *** Waiting for unfinished jobs....
clang-9: error: cannot specify -o when generating multiple output files
../lib.mk:152: recipe for target 'xxx/tools/testing/selftests/bpf/test_progs' failed
Signed-off-by: NStanislav Fomichev <sdf@google.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/Makefile

@@ -209,7 +209,7 @@ ifeq ($(DWARF2BTF),y)
 endif
 
 PROG_TESTS_H := $(OUTPUT)/prog_tests/tests.h
-$(OUTPUT)/test_progs: $(PROG_TESTS_H)
+test_progs.c: $(PROG_TESTS_H)
 $(OUTPUT)/test_progs: CFLAGS += $(TEST_PROGS_CFLAGS)
 $(OUTPUT)/test_progs: prog_tests/*.c
 
@@ -232,7 +232,7 @@ $(PROG_TESTS_H): $(PROG_TESTS_DIR) $(PROG_TESTS_FILES)
 		 ) > $(PROG_TESTS_H))
 
 VERIFIER_TESTS_H := $(OUTPUT)/verifier/tests.h
-$(OUTPUT)/test_verifier: $(VERIFIER_TESTS_H)
+test_verifier.c: $(VERIFIER_TESTS_H)
 $(OUTPUT)/test_verifier: CFLAGS += $(TEST_VERIFIER_CFLAGS)
 
 VERIFIER_TESTS_DIR = $(OUTPUT)/verifier