Remove zlib.h inclusion from trace.h

When zlib.h is included in trace.h, the following inclusion chain
exists:

  Defn.h -> trace.h -> zlib.h -> zconf.h -> unistd.h

Since R defines many of its global variables by redefining "extern" to
an empty symbol and including Defn.h into main.c, this will declare
(among others) "char **environ" in main.c, overwriting the variable of
the same name from the C library. This causes a segmentation fault
when the tcltk library is loaded, e.g. when install.packages is
invoked.