remove gcc_unused

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/input/InputStream.cxx

@@ -69,7 +69,7 @@ InputStream::CheapSeeking() const noexcept
 }
 
 void
-InputStream::Seek(std::unique_lock<Mutex> &, gcc_unused offset_type new_offset)
+InputStream::Seek(std::unique_lock<Mutex> &, [[maybe_unused]] offset_type new_offset)
 {
 	throw std::runtime_error("Seeking is not implemented");
 }