LaTeX export: Fix interpretation of \nbsp

Reported by Chris Gray.

lisp/org-latex.el

@@ -1731,6 +1731,7 @@ The conversion is made depending of STRING-BEFORE and STRING-AFTER."
    "\\medskip"
    "\\multicolumn"
    "\\multiput"
+   "\\nbsp"
    "\\newcommand"
    "\\newcounter"
    "\\newenvironment"