The relevant question is, under which license they provided their code.
They may implicitly have provided it under the license of the project, but this may be not that clear and unclarity may result in legal problems. Very minor corrections may not have a copyright at all, e.g., when someone points out a typo there is probably not much to do.
The other answer explains how to add the code when it is GPL (compatible) licensed.
In theory someone could send a patch without any license (thus not granting the right to use it at all) or with an incompatible license, their code is at first just a piece of text that is copyrighted by them. Using it together with your code may be prohibited in this case, what would make it mostly useless.
Useful licenses for them would include GPLv3 and compatible. They can, for example, decide to make it available under BSDL. This would mean, that the combination of your product and their code would only be usable under GPL, but their code alone can be used together with other projects without the restrictions imposed by the GPL.
Another interesting aspect is, that the GPL is about distribution. Even when the code may not be distributed together with your product, you (and they) can use the combined product yourself as long as you do not distribute it.
That's the theory. Now for the practical aspect: Ask them to tell you under which license it can be used and suggest to use a GPLv3 compatible one explaining that you otherwise cannot include it into the product.
From the discussion: A simple case in which a patch may have an unclear, more permissive, or more prohibitive license would be that your GPLed program is a fork of a BSDL program.
Now I may have written a patch that applies to the BSDL program and can be used with your GPLed program as well because let's say it did not modify that file after the fork.
Following the BSDL, I can both have my patch BSDL, and you can still add it to your program or have it under a proprietary license that still follows the BSDL terms. The BSDL does not prohibit the use in proprietary codes but only has some less restrictive requirements like crediting the authors. (Read the license for full details)
In the second case, you're not allowed to combine it with your code. Combining it with the pre-fork code would probably make the derived work licensed under my proprietary license or just be disallowed, depending on the terms of my more restrictive license.
So the final recommendation is: Make sure to have people tell you how their code can be used.
In most cases, it will be something like "It can be used under your project's license" or "You are free to use it however you like" anyway.
