Merge bitcoin#640: scalar_impl.h: fix includes

2cb73b1 scalar_impl.h: fix includes (Marko Bencun)

Pull request description:

  group.h functions are not referenced.
  utils.h added as functions like VERIFY_CHECK are used.

ACKs for commit 2cb73b:
  sipa:
    ACK 2cb73b1

Tree-SHA512: b9c7367061c2a22d2c9266c61261edd47798551b03b878ecd2e005d858701487145589793406cb4e88e85cd3c769007132efac9c228d5ee288e487e7d308e1c2

src/scalar_impl.h

@@ -7,8 +7,8 @@
 #ifndef SECP256K1_SCALAR_IMPL_H
 #define SECP256K1_SCALAR_IMPL_H
 
-#include "group.h"
 #include "scalar.h"
+#include "util.h"
 
 #if defined HAVE_CONFIG_H
 #include "libsecp256k1-config.h"