Fix Luacheck warnings (#1798)

This fixes warnings that were introduced with f22ef5014d /
https://github.com/awesomeWM/awesome/pull/1477.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/beautiful/xresources.lua

@@ -91,8 +91,8 @@ function xresources.get_dpi(s)
         if not xresources.dpi then
             if root then
                 local mm_to_inch = 25.4
-                _, h = root.size()
-                _, hmm = root.size_mm()
+                local _, h = root.size()
+                local _, hmm = root.size_mm()
                 if hmm ~= 0 then
                     xresources.dpi = round(h*mm_to_inch/hmm)
                 end