Add missing reactfx dependency

This had been getting pulled in transitively through the old
io.bisq.exchange:core artifact's parent pom. Now that we're depending on
the io.bisq:bisq-core, there is no parent pom in the mix anymore, and
the reactfx dependency must to be explicitly declared here.

pom.xml

@@ -368,6 +368,12 @@
             <version>8.0.6_20</version>
         </dependency>
 
+        <dependency>
+            <groupId>org.reactfx</groupId>
+            <artifactId>reactfx</artifactId>
+            <version>2.0-M3</version>
+        </dependency>
+
         <dependency>
             <groupId>net.glxn</groupId>
             <artifactId>qrgen</artifactId>