awesomerc: use gmatch instead of deprecated gfind

a user in #awesome made me aware of a little quirk in lib/awful.lua
which renders the history of the prompt unusable when using Gentoo and
installing Lua without the USE-flag "deprecated". Lua states that
string.gfind has been replaced with string.gmatch and aborts the
function prompt_history_save(id). I attached a patch that replaces
id:gfind with id:gmatch, which so far has not revealed any problems.

Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful.lua.in

@@ -881,7 +881,7 @@ local function prompt_history_save(id)
         local f = io.open(id, "w")
         if not f then
             local i = 0
-            for d in id:gfind(".-/") do
+            for d in id:gmatch(".-/") do
                 i = i + #d
             end
             mkdir(id:sub(1, i - 1))