applypatch: Forward declare struct Value.

And move '#include "edify/expr.h"' into .cpp files. This breaks the
transitive dependency on libedify. Modules that include
"applypatch/applypatch.h" don't need to add libedify into their
dependency list, unless they really need anything from libedify.

Build libedify static library for host, which is needed by
libimgpatch.

Test: mmma bootable/recovery
Change-Id: Ibb53d322579fcbf593438d058d9bcee240625941

applypatch/include/applypatch/applypatch.h

@@ -27,7 +27,8 @@
 
 #include <openssl/sha.h>
 
-#include "edify/expr.h"
+// Forward declaration to avoid including "edify/expr.h" in the header.
+struct Value;
 
 struct FileContents {
   uint8_t sha1[SHA_DIGEST_LENGTH];