Center longtable explicitly (#10690)

"Center" is the default of longtable, but some situations require it
to be centered explicitly, such as when using floatrow:

https://tex.stackexchange.com/questions/320592/floatrow-package-conflicting-with-centering-longtable

src/insets/InsetTabular.cpp

@@ -2777,6 +2777,7 @@ void Tabular::latex(otexstream & os, OutputParams const & runparams) const
 			os << "[l]";
 			break;
 		case LYX_LONGTABULAR_ALIGN_CENTER:
+			os << "[c]";
 			break;
 		case LYX_LONGTABULAR_ALIGN_RIGHT:
 			os << "[r]";