Update build.sh (#2068)

fix mkwork compilation errors

attic/world/build.sh

@@ -1,3 +1,3 @@
 #!/bin/bash
 
-c++ -std=c++11 -I../.. -I.. -g -o mkworld ../../node/C25519.cpp ../../node/Salsa20.cpp ../../node/SHA512.cpp ../../node/Identity.cpp ../../node/Utils.cpp ../../node/InetAddress.cpp ../../osdep/OSUtils.cpp mkworld.cpp -lm
+c++ -std=c++11 -I../.. -I../../ext -I.. -g -o mkworld ../../node/C25519.cpp ../../node/Salsa20.cpp ../../node/SHA512.cpp ../../node/Identity.cpp ../../node/Utils.cpp ../../node/InetAddress.cpp ../../osdep/OSUtils.cpp mkworld.cpp -lm