zLinux build error with gcc and IBM Java headers (facebook#4013)

Summary:
`SetByteArrayRegion` does not have const source buffer thus compilation error. I have made that same as in other JNI files (const_cast). It was missing for new transaction functionality added recently.
Closes facebook#4013

Differential Revision: D8493290

Pulled By: sagar0

fbshipit-source-id: 14afedf365b111121bd11e68a8d546a1cae68b26

java/rocksjni/transaction.cc

@@ -183,7 +183,7 @@ jbyteArray txn_get_helper(JNIEnv* env, const FnGet& fn_get,
       return nullptr;
     }
     env->SetByteArrayRegion(jret_value, 0, static_cast<jsize>(value.size()),
-                            reinterpret_cast<const jbyte*>(value.c_str()));
+                            const_cast<jbyte*>(reinterpret_cast<const jbyte*>(value.c_str())));
     if (env->ExceptionCheck()) {
       // exception thrown: ArrayIndexOutOfBoundsException
       return nullptr;
@@ -350,7 +350,7 @@ jobjectArray txn_multi_get_helper(JNIEnv* env, const FnMultiGet& fn_multi_get,
 
       env->SetByteArrayRegion(
           jentry_value, 0, static_cast<jsize>(value_parts[i].size()),
-          reinterpret_cast<const jbyte*>(value_parts[i].c_str()));
+          const_cast<jbyte*>(reinterpret_cast<const jbyte*>(value_parts[i].c_str())));
       if (env->ExceptionCheck()) {
         // exception thrown: ArrayIndexOutOfBoundsException
         env->DeleteLocalRef(jentry_value);