According to Free Software Foundation's lawyers copyright does not apply to header files:
I'm not sure what your project is designed to do, so I don't have an
opinion about how it stands regarding the GPL. However, I've talked
with our lawyer about one specific issue that you raised: that of
using simple material from header files.
Someone recently made the
claim that including a header file always makes a derivative work.
That's not the FSF's view. Our view is that just using structure
definitions, typedefs, enumeration constants, macros with simple
bodies, etc., is NOT enough to make a derivative work. It would take
a substantial amount of code (coming from inline functions or macros
with substantial bodies) to do that. (source: email by RMS on LKML)
There is legal precedence supporting that. Oracle's lawyers attempted to claim that APIs (and header files defining the APIs) are copyrightable, yet the court has disagreed with them (emphasis mine).
However, on the primary copyright issue of the APIs, the court ruled
that "So long as the specific code used to implement a method is
different, anyone is free under the Copyright Act to write his or her
own code to carry out exactly the same function or specification of
any methods used in the Java API. It does not matter that the
declaration or method header lines are identical." The ruling found
that the structure Oracle was claiming was not copyrightable under
section 102(b) of the Copyright Act because it was a "system or method
of operation." (source: Oracle vs Google)