Make linker include library ctor/dtor in statically linked libisc

Make linker include library ctor/dtor in statically linked libisc

Under normal situation, the linker throws out all symbols from
compilation unit when no symbols are used in the final binary, which is
the case for lib/isc/lib.c.  This commit adds empty function to lib.c
that's being called from different CU (mem.c in this case) and that
makes the linker to include all the symbols including the normally
unreferenced isc__initialize() and isc__shutdown() in the final binary.