[FIX] odoo.sty: fix compatibility with older sphinx versions

`\sphinxmaketitle` only exists as of v1.8.3, and `\renewcommand` would
crash when the command is not defined. Similarly `\newcommand` would
crash if the command already exists.

`\providecommand` acts as a shim: no-op if the command exists,
otherwise creates it so that it can be redefined.

Backport of 54e7ed30ac

_static/latex/odoo.sty

@@ -97,6 +97,7 @@
 
 % For sphinx 1.8.3+, the command is \sphinxmaketitle and the
 % layout is slightly different on the titlepage
+\providecommand\sphinxmaketitle{}
 \renewcommand{\sphinxmaketitle}{%
   \noindent\rule{\textwidth}{1pt}\par
   \begin{flushright}