build.gradle: Version control generated protobuf files again.

This effectively reverts commit 002fe8184c. It lead to too much support hassle.

core/build.gradle

@@ -24,6 +24,10 @@ dependencies {
 
 sourceCompatibility = 1.7
 
+protobuf {
+    generatedFilesBaseDir = new File(projectDir, '/src') // workaround for '$projectDir/src'
+}
+
 test {
     exclude 'org/bitcoinj/net/NetworkAbstractionTests*'
 }