Fix building OpenDingux images

The icons appear to be referenced incorrectly not not valid for the
device.

Packaging/OpenDingux/gkd350h.desktop

@@ -5,7 +5,7 @@ Exec=devilutionx
 Terminal=false
 Type=Application
 StartupNotify=true
-Icon=Diablo_32
+Icon=icon
 Categories=games;
 X-OD-Manual=readme.gcw0.txt
 X-OD-NeedsDownscaling=true

Packaging/OpenDingux/package-opk.sh

@@ -15,7 +15,7 @@ package_opk() {
 	cp "Packaging/OpenDingux/${TARGET}-manual.txt" "${tmp}/readme.${ext}.txt"
 	mksquashfs "${BUILD_DIR}/devilutionx" \
 		"${tmp}/default.${ext}.desktop" "${tmp}/readme.${ext}.txt" \
-		Packaging/resources/Diablo_32.png Packaging/resources/CharisSILB.ttf \
+		Packaging/resources/icon.png Packaging/resources/CharisSILB.ttf \
 		"${BUILD_DIR}/devilutionx-${TARGET}.opk" \
 		-all-root -no-xattrs -noappend -no-exports -no-progress
 }

Packaging/OpenDingux/retrofw.desktop

@@ -5,7 +5,7 @@ Exec=devilutionx
 Terminal=false
 Type=Application
 StartupNotify=true
-Icon=Diablo_32
+Icon=icon
 Categories=games;
 X-OD-Manual=readme.retrofw.txt
 X-OD-NeedsDownscaling=true

Packaging/OpenDingux/rg350.desktop

@@ -5,7 +5,7 @@ Exec=devilutionx
 Terminal=false
 Type=Application
 StartupNotify=true
-Icon=Diablo_32
+Icon=icon
 Categories=games;
 X-OD-Manual=readme.gcw0.txt
 X-OD-NeedsDownscaling=true