maybe.h: reference header new

This is required for the usage of placement new. Not an issue for fish
as it gets picked up from elsewhere, but it lets one use it in a C++
test directly this way.

src/maybe.h

@@ -2,6 +2,7 @@
 #define FISH_MAYBE_H
 
 #include <cassert>
+#include <new>
 #include <type_traits>
 #include <utility>