[build] Fix make distcheck

A regression from 7b77ce0507.
It was caught by the CI build in the PR, but apparently we are now used
to CI failures that they are effectively ignored.

Also, yay for multiple build systems.

test/subset/data/Makefile.am

@@ -19,6 +19,7 @@ EXTRA_DIST += \
 	expected/layout.gpos4 \
 	expected/layout.gpos6 \
 	expected/layout.gpos8 \
+	expected/layout.gpos8.amiri \
 	expected/layout.gsub3 \
 	expected/layout.gsub6 \
 	expected/layout.gdef \