CMakeLists: Make libzip excluded from the ALL target

Likewise, we also only want to link in the libraries that we actually
make use of (so we don't need to worry about linking in test targets).

externals/CMakeLists.txt

@@ -81,7 +81,7 @@ endif()
 add_subdirectory(zlib EXCLUDE_FROM_ALL)
 
 # libzip
-add_subdirectory(libzip)
+add_subdirectory(libzip EXCLUDE_FROM_ALL)
 
 if (ENABLE_WEB_SERVICE)
     # LibreSSL