Fix missing spring-instrument jar manifest entry

The spring-intstrument jar should have a 'Premain-Class:' manifest
entry in order to enable use as a Java agent. This entry was present in
versions 3.1.1 and earlier, but due to build infrastructure changes
starting in 3.2.x this entry was missed. It is now back in place as
expected.

Issue: SPR-9458

build.gradle

@@ -183,6 +183,10 @@ project('spring-instrument') {
     dependencies {
         compile project(":spring-core")
     }
+    jar {
+        manifest.attributes['Premain-Class'] =
+            'org.springframework.instrument.InstrumentationSavingAgent'
+    }
 }
 
 project('spring-instrument-tomcat') {