Fix compilation with clang/libc++

The issue was that redeclaring the hash template in the code somehow seems to shadow the declaration that is done in <functional> and <memory> and afterwards the template specialization of the hash function seems to use our redefined template instead of libc++’s one (most likely due to the fact that libc++ uses an inline namespace inside std)

include/fc/array.hpp

@@ -117,8 +117,6 @@ namespace fc {
 #include <fc/crypto/city.hpp>
 namespace std
 {
-    template<typename T> struct hash;
-
     template<typename T, size_t N>
     struct hash<fc::array<T,N> >
     {

include/fc/crypto/ripemd160.hpp

@@ -79,8 +79,6 @@ class ripemd160
 
 namespace std
 {
-    template<typename T> struct hash;
-
     template<>
     struct hash<fc::ripemd160>
     {

include/fc/crypto/sha1.hpp

@@ -72,8 +72,6 @@ class sha1
 
 namespace std
 {
-    template<typename T> struct hash;
-
     template<>
     struct hash<fc::sha1>
     {

include/fc/crypto/sha224.hpp

@@ -73,8 +73,6 @@ class sha224
 } // fc
 namespace std
 {
-    template<typename T> struct hash;
-
     template<>
     struct hash<fc::sha224>
     {

include/fc/network/ip.hpp

@@ -112,8 +112,6 @@ namespace fc {
 }
 namespace std
 {
-    template<typename T> struct hash;
-
     template<>
     struct hash<fc::ip::endpoint>
     {

include/fc/uint128.hpp

@@ -99,8 +99,6 @@ namespace fc
 
 namespace std
 {
-    template<typename T> struct hash;
-
     template<>
     struct hash<fc::uint128>
     {