d/control: add build dep to python-sphinx-rtd-theme

d/control: add build dep to python-sphinx-rtd-theme

Avoids the following issue in sid:
  Theme error:
  sphinx_rtd_theme is no longer a hard dependency since version 1.4.0.
  Please install it manually.(pip install sphinx_rtd_theme)
  /<<PKGBUILDDIR>>/mk/rte.sdkdoc.mk:110: recipe for target 'guides-html'
  failed
  make[2]: *** [guides-html] Error 1
  /<<PKGBUILDDIR>>/mk/rte.sdkroot.mk:107: recipe for target
  'doc-guides-html' failed
  make[1]: *** [doc-guides-html] Error 2
  make[1]: *** Waiting for unfinished jobs...

Change-Id: Ia5662a7112b9fbf63a40db5ba7527c156b6536b8
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>