Put utf8.h in the correct place

Both CMake and Nix want it to be here, so I obliged.

depends/packages/utfcpp.mk

@@ -6,5 +6,6 @@ $(package)_download_file=v$($(package)_version).tar.gz
 $(package)_sha256_hash=8d6aa7d77ad0abb35bb6139cb9a33597ac4c5b33da6a004ae42429b8598c9605
 
 define $(package)_stage_cmds
-  cp -a ./source $($(package)_staging_dir)$(host_prefix)/include
+  mkdir -p $($(package)_staging_dir)$(host_prefix)/include && \
+  cp -a ./source $($(package)_staging_dir)$(host_prefix)/include/utf8cpp
 endef

src/wallet/rpcwallet.cpp

@@ -46,7 +46,7 @@
 
 #include <boost/assign/list_of.hpp>
 #include <boost/algorithm/string/trim.hpp>
-#include <utf8.h>
+#include <utf8cpp/utf8.h>
 
 #include <univalue.h>