selftest/memfd/Makefile: Fix build error

Selftest for memfd shows build error as below:
```
gcc -D_FILE_OFFSET_BITS=64 -I../../../../include/uapi/ -I../../../../include/ -I../../../../usr/include/    fuse_mnt.c  -o /home/sjpark/linux/tools/testing/selftests/memfd/fuse_mnt
/tmp/cc6NHdwJ.o: In function `main':
fuse_mnt.c:(.text+0x249): undefined reference to `fuse_main_real'
collect2: error: ld returned 1 exit status
```

The build fails because output file is specified without $(OUTPUT) and
LDFLAGS is used though Makefile implicit rule is used.  This commit
fixes the error by specifying output file path with $(OUTPUT) and using
LDLIBS instead of LDFLAGS.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>