diffutils: add cookbook_make and cookbook_make_install steps

The diffutils recipe only ran cookbook_configure but never actually
compiled or installed anything, leaving stage.tmp empty (4K). This
caused redbear-mini to fail with a 4.0K INCOMPLETE stage and no
diff/diff3 binaries in the final image.

Add the missing make and make install steps, matching the pattern
used by other autotools recipes (bison, findutils, etc.).

local/recipes/tools/diffutils/recipe.toml

@@ -26,6 +26,8 @@ COOKBOOK_CONFIGURE_FLAGS+=(
     ac_cv_header_stdio_h=yes
 )
 cookbook_configure
+"${COOKBOOK_MAKE}" -j "${COOKBOOK_MAKE_JOBS}"
+"${COOKBOOK_MAKE}" install DESTDIR="${COOKBOOK_STAGE}"
 rm -f "${COOKBOOK_STAGE}/usr/share/info/dir"
 """