docs: Fix Sphinx detection with out-of-tree builds

Adapt to changes made in r302499.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302517 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CMakeLists.txt

@@ -1,9 +1,9 @@
 
 if (LLVM_ENABLE_SPHINX)
+  include(AddSphinxTarget)
   if (SPHINX_FOUND)
-    include(AddSphinxTarget)
     if (${SPHINX_OUTPUT_HTML})
       add_sphinx_target(html libcxx)
     endif()
   endif()
-endif()
+endif()