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/bspatch.cpp

@@ -30,6 +30,7 @@
 #include <openssl/sha.h>
 
 #include "applypatch/applypatch.h"
+#include "edify/expr.h"
 #include "print_sha1.h"
 
 void ShowBSDiffLicense() {
@@ -91,4 +92,4 @@ int ApplyBSDiffPatch(const unsigned char* old_data, size_t old_size, const Value
     }
   }
   return result;
-}
+}