stap blacklist -= sys_execve

[Jim Keniston]

As part of the wrapup for PR #1345, I've removed sys_execve from the
return-probes blacklist in tapsets.cxx.  This means stap will no longer
silently avoid setting a return probe on this function.  This should be
no problem unless you're using RHEL4 U2, or v2.6.15-rc2 or earlier of
the mainstream kernel.

Jim