Add 'Fragment-Host' to submodules for OSGi

Based on discussion on issue ReactiveX/RxJava#154

build.gradle

@@ -82,5 +82,6 @@ jar {
         instruction 'Bundle-Vendor', 'Netflix'
         instruction 'Bundle-DocURL', 'https://github.com/Netflix/RxJava'
         instruction 'Import-Package', '!org.junit,!junit.framework,!org.mockito.*,*'
+        instruction 'Fragment-Host', 'com.netflix.rxjava.core'
     }
 }