Move explicit instantiation of IncrementalMerkleTree::emptyroots into header

Fixes a -Wundefined-var-template warning with clang.

src/zcash/IncrementalMerkleTree.cpp

@@ -45,9 +45,6 @@ public:
 template<size_t Depth, typename Hash>
 EmptyMerkleRoots<Depth, Hash> PathFiller<Depth, Hash>::emptyroots;
 
-template<size_t Depth, typename Hash>
-EmptyMerkleRoots<Depth, Hash> IncrementalMerkleTree<Depth, Hash>::emptyroots;
-
 template<size_t Depth, typename Hash>
 void IncrementalMerkleTree<Depth, Hash>::wfcheck() const {
     if (parents.size() >= Depth) {

src/zcash/IncrementalMerkleTree.hpp

@@ -216,6 +216,9 @@ public:
     static SHA256Compress combine(const SHA256Compress& a, const SHA256Compress& b);
 };
 
+template<size_t Depth, typename Hash>
+EmptyMerkleRoots<Depth, Hash> IncrementalMerkleTree<Depth, Hash>::emptyroots;
+
 } // end namespace `libzcash`
 
 typedef libzcash::IncrementalMerkleTree<INCREMENTAL_MERKLE_TREE_DEPTH, libzcash::SHA256Compress> ZCIncrementalMerkleTree;