applypatch: Compile libimgpatch for target and host.

update_engine need it for the new IMGDIFF operation.

Also removed __unused in ApplyImagePatch() as I got error building it
for the host, and I think it's dangerous not checking the size of the
input.

Test: mma
Bug: 26628339

Change-Id: I22d4cd55c2c3f87697d6afdf10e8106fef7d1a9c

applypatch/include/applypatch/imgpatch.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef _IMGPATCH_H
+#define _IMGPATCH_H
+
+typedef ssize_t (*SinkFn)(const unsigned char*, ssize_t, void*);
+
+int ApplyImagePatch(const unsigned char* old_data, ssize_t old_size,
+                    const unsigned char* patch_data, ssize_t patch_size,
+                    SinkFn sink, void* token);
+
+#endif  //_IMGPATCH_H