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 <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/memfd/Makefile

···
 TEST_GEN_FILES := memfd_test fuse_mnt fuse_test
 
 fuse_mnt.o: CFLAGS += $(shell pkg-config fuse --cflags)
-fuse_mnt: LDFLAGS += $(shell pkg-config fuse --libs)
 
 include ../lib.mk
 
+$(OUTPUT)/fuse_mnt: LDLIBS += $(shell pkg-config fuse --libs)