This should normally patch Makefile.am and use autotools.eclass to re-run
autoreconf. However, this does not work out of the box and pulling out the big
artillery to fix a simple typo which will be gone in the next release wasn't
worth the trouble.

See https://bugs.gentoo.org/542604.

Denis Dupeyron <calchan@gentoo.org>

--- icons/128x128/Makefile.in.orig	2015-04-20 21:16:04.376497095 -0600
+++ icons/128x128/Makefile.in	2015-04-20 21:16:23.423138219 -0600
@@ -337,7 +337,7 @@
 top_build_prefix = @top_build_prefix@
 top_builddir = @top_builddir@
 top_srcdir = @top_srcdir@
-iconsdir = $(datadir)/icons/hicolor/48x48/apps
+iconsdir = $(datadir)/icons/hicolor/128x128/apps
 icons_DATA = ristretto.png
 EXTRA_DIST = $(icons_DATA)
 all: all-am