syscalls/ipc/Makefile.inc: changed LIB to LIBIPC
Both lib.mk and testcases/kernel/syscalls/ipc/Makefile.inc set
LIB. It confuses make when both failes are included into a leaf Makefile.
Renamed LIB to LIBIPC in Makefile.inc.
The idea is by Cyril Hrubis <email@example.com>
Signed-off-by: Stanislav Kholmanskikh <firstname.lastname@example.org>