selftests/bpf : clean up feature/ when make clean

An error "implicit declaration of function 'reallocarray'" can be thrown
with the following steps:

$ cd tools/testing/selftests/bpf
$ make clean && make CC=<Path to GCC 4.8.5>
$ make clean && make CC=<Path to GCC 7.x>

The cause is that the feature folder generated by GCC 4.8.5 is not
removed, leaving feature-reallocarray being 1, which causes reallocarray
not defined when re-compliing with GCC 7.x. This diff adds feature
folder to EXTRA_CLEAN to avoid this problem.

v2: Rephrase the commit message.
Signed-off-by: Hechao Li <hechaol@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/Makefile

@@ -279,4 +279,5 @@ $(OUTPUT)/verifier/tests.h: $(VERIFIER_TESTS_DIR) $(VERIFIER_TEST_FILES)
 		 ) > $(VERIFIER_TESTS_H))
 
 EXTRA_CLEAN := $(TEST_CUSTOM_PROGS) $(ALU32_BUILD_DIR) \
-	$(VERIFIER_TESTS_H) $(PROG_TESTS_H) $(MAP_TESTS_H)
+	$(VERIFIER_TESTS_H) $(PROG_TESTS_H) $(MAP_TESTS_H) \
+	feature