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

[xgupta]

Summary

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

[xgupta]

Error Message was:

CMake Error at /home/xgupta/dev/llvm-project/llvm/cmake/modules/AddSphinxTarget.cmake:71 (add_dependencies):

Cannot add target-level dependencies to non-existent target "sphinx".

The add_dependencies works for top-level logical targets created by the
add_executable, add_library, or add_custom_target commands.  If you want to
add file-level dependencies see the DEPENDS option of the add_custom_target
and add_custom_command commands.

Call Stack (most recent call first):

/home/xgupta/dev/llvm-project/libunwind/docs/CMakeLists.txt:5 (add_sphinx_target)

[compnerd]

I suppose that this makes it difficult to generate them when libunwind is built standalone, but, as it is, there are other dependencies involved.

libunwind/docs/CMakeLists.txt

 include(FindSphinx)
-if (SPHINX_FOUND)
+if (SPHINX_FOUND AND LLVM_ENABLE_SPHINX)
   include(AddSphinxTarget)
   if (${SPHINX_OUTPUT_HTML})
     add_sphinx_target(html libunwind)
   endif()
 endif()