selftests: Fix memfd to support relocatable build (O=objdir)

Fix memfd to support relocatable build (O=objdir). This calls out
source files necessary to build tests and simplfies the dependency
enforcement.

Tested the following:

Note that cross-build for fuse_mnt has dependency on -lfuse.

make all
make clean
make kselftest-install O=/arm64_build/ ARCH=arm64 HOSTCC=gcc \
CROSS_COMPILE=aarch64-linux-gnu- TARGETS=memfd

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>