[NFC][libunbind] Fix Sphinx error during CMake invocation

An error has occurred when I build libunwind with -DLLVM_BUILD_DOCS=ON.

Reviewed By: #libunwind, compnerd

Differential Revision: https://reviews.llvm.org/D96107

libunwind/docs/CMakeLists.txt

@@ -1,5 +1,5 @@
 include(FindSphinx)
-if (SPHINX_FOUND)
+if (SPHINX_FOUND AND LLVM_ENABLE_SPHINX)
   include(AddSphinxTarget)
   if (${SPHINX_OUTPUT_HTML})
     add_sphinx_target(html libunwind)