workaround a type handling bug in SystemTap Dtrace 1.2

workaround a type handling bug in SystemTap Dtrace 1.2

It uses a construct similar to:

  __typeof__((func)) x = func;

but hek_key is a char[1] so the above becomes:

  char arg1[1] = func;

which is invalid.

Using a temporary allows an implicit conversion to const char * and
avoids possible bugs that might be hidden by an explicit cast.