system/FileDescriptor: add operator!=

src/system/FileDescriptor.hxx

@@ -65,6 +65,10 @@ public:
 		return fd == other.fd;
 	}
 
+	constexpr bool operator!=(FileDescriptor other) const noexcept {
+		return !(*this == other);
+	}
+
 	constexpr bool IsDefined() const noexcept {
 		return fd >= 0;
 	}