add a .gitignore in the T directory

git status was way too noisy

common/utils/T/.gitignore

+*.o
+*.a
+T_IDs.h
+T_messages.txt.h
+genids
+tracer/enb
+tracer/extract_config
+tracer/record
+tracer/replay
+tracer/textlog
+tracer/vcd
+tracee/tracee