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 <[e-mail]>
Signed-off-by: Anders Roxell <[e-mail]>
Signed-off-by: Daniel Díaz <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/sync/Makefile

···
 	$(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 $@