maintainer for xpressive is eric niebler

[SVN r31205]

libs/maintainers.txt

@@ -58,3 +58,4 @@ typeof                Arkadiy Vertleyb <vertleyb -at- hotmail.com>, Peder Holt <
 utility
 variant               Eric Friedman <ericbrandon -at- gmail.com>
 wave                  Hartmut Kaiser <hartmut.kaiser -at- gmail.com>
+xpressive             Eric Niebler <eric -at- boost-consulting.com>