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>