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/thread/Name.hxx

@@ -58,7 +58,7 @@ SetThreadName(const char *name) noexcept
 
 template<typename... Args>
 static inline void
-FormatThreadName(const char *fmt, gcc_unused Args&&... args) noexcept
+FormatThreadName(const char *fmt, [[maybe_unused]] Args&&... args) noexcept
 {
 #ifdef HAVE_THREAD_NAME
 	SetThreadName(StringFormat<16>(fmt, args...));