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>

test/run_inotify.cxx

@@ -34,8 +34,8 @@ static constexpr unsigned IN_MASK =
 	|IN_MOVE|IN_MOVE_SELF;
 
 static void
-my_inotify_callback(gcc_unused int wd, unsigned mask,
-		    const char *name, gcc_unused void *ctx)
+my_inotify_callback([[maybe_unused]] int wd, unsigned mask,
+		    const char *name, [[maybe_unused]] void *ctx)
 {
 	printf("mask=0x%x name='%s'\n", mask, name);
 }