add Pod::To::HTML dependency

It's used by ./htmlify.p6

Author: dwarring

META.info

@@ -2,7 +2,7 @@
     "name"        : "p6doc",
     "version"     : "*",
     "description" : "Perl 6 documentation (tools and docs)",
-    "depends"     : [ "URI", "File::Temp", "JSON::Fast", "Pod::To::BigPage" ],
+    "depends"     : [ "URI", "File::Temp", "JSON::Fast", "Pod::To::BigPage", "Pod::To::HTML" ],
     "build-depends" : [ "File::Find" ],
     "provides"    : {
         "Perl6::Type"                   : "lib/Perl6/Type.pm",