As jkj says in the comment above, the problem is that you appear to be trying to patch a binary. I apologise for simply repeating his/her comment, but I decided that if it was right, merely upvoting it wasn't emphatic enough, because what you're doing will never work.
That's not how patches function. Patches are text changes that are applied to an existing piece of text, in this case the source for mysqldump, so that the binary can be rebuilt as part of the normal build.
You don't say how you got the version of mysql you're running, but assuming that it's from a package rather than from source - as would be normal on CentOS - you've got an awful lot of work to do. First, you'll need to build mysql from source, and replace the existing packaged version (or, for much less pain, build it to install alongside in a custom directory). Then you'll have to verify that the newly-built mysql does what you expect. Then you'll have to patch the sources, and rebuild the binaries.
The reason for the verification step above is because of Red Hat's patching policy, which is that they will often include fixes for both the version of something they provide, and indeed they will backport useful fixes from later versions, without bumping the version number. The corollary of this is that you can't be sure that RH's mysql 5.5.20 is anything like MySQL's 5.5.20.
Indeed, you might do better to get the mysql SRPM (RPM of sources) from redhat, install that, and check by eye to see if the patch you want is already included. Even if it's not, the SRPM will give you a much closer approximation to the correct sources to reproduce the binaries you have installed. The downside is that if RH have made any other changes to mysqldump.c, the patch will no longer apply cleanly, and you will need some knowledge of the code to get it to do so without wildly breaking things.
If this is all known to you, I apologise for laying it out. If it's not, I thought it might be best to be very clear and upfront just how much pain you're in for if you decide to start building your own binaries.