Removed unused include directory lib/T2Hack from Makefile in NxFloat and TestSerial apps

This is a small patch to remove a non-existing include directory from the Makefile of apps/tests/NxFloat and apps/tests/TestSerial:

CFLAGS += -I$(TOSDIR)/lib/T2Hack

Both applications try to include $(TOSDIR)/lib/T2Hack which does not exist in tinyos-main.