testcases/kernel/mem: Fix compilation failure

When compiling, the following error occurred:

make[6]: Entering directory `/nfs/hosts/sestonas06/homes/mlil/git/ltp/testcases/kernel/mem/lib'
make[6]: ../../../../../testcases/kernel/mem/lib/Makefile: No such file or directory

This was caused by testcases/kernel/mem/include/libmem.mk that used
relative path to the $(LIBMEM_SRCDIR)/Makefile while changing working
directory (with make -C) at the same time.

This could be reproduced by:

1. cd testcases/kernel/mem/lib
2. rm libmem.a
3. cd ../hugetlb/hugemmap
4. make

Reported-by: Mats Liljegren <mats.liljegren@enea.com>
Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
Tested-by: Mats Liljegren <mats.liljegren@enea.com>