selftests: sync: missing CFLAGS while compiling

Based on patch: https://patchwork.kernel.org/patch/10042045/

arch64-linux-gnu-gcc -c sync.c -o sync/sync.o
sync.c:42:29: fatal error: linux/sync_file.h: No such file or directory
 #include <linux/sync_file.h>
                             ^
CFLAGS is not used during the compile step, so the system instead of
kernel headers are used.  Fix this by adding CFLAGS to the OBJS compile
rule.
Reported-by: Lei Yang <Lei.Yang@windriver.com>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/sync/Makefile

@@ -30,7 +30,7 @@ $(TEST_CUSTOM_PROGS): $(TESTS) $(OBJS)
 	$(CC) -o $(TEST_CUSTOM_PROGS) $(OBJS) $(TESTS) $(CFLAGS) $(LDFLAGS)
 
 $(OBJS): $(OUTPUT)/%.o: %.c
-	$(CC) -c $^ -o $@
+	$(CC) -c $^ -o $@ $(CFLAGS)
 
 $(TESTS): $(OUTPUT)/%.o: %.c
 	$(CC) -c $^ -o $@