Indeed, the default license is “all rights reserved”. On the other hand, facts are not copyrightable.
If any reasonable representation of these facts would result in the essentially same header file, there is an argument that there is nothing copyrightable in the header file (compare the US merger doctrine concept, though there's similar case law in the EU). Copyrightable aspects might also involve the structure and organization of the file. In particular, if you're in a jurisdiction that recognizes database rights (such as the EU), a collection of facts can be copyrightable as well.
Personally, if this was as fairly small snippet (e.g. < 10 lines) I'd probably feel sufficiently comfortable using it without any license. For example, I've often copied code snippets that have no reasonable other representation like “array with the first 100 primes” or “declaration for a PI constant”. Just yesterday, I “plagiarized” the structure of a C assert() macro from the GNU libc.
In your particular example, you could likely take the frequencies from publicly available sources such as Wikipedia and write your own header file, avoiding all of these concerns. Yes, Wikipedia is also copyrighted, but you would merely use it as a source of facts.