pkg/debian: Explicitly set module compression to xz

pkg/debian/kernel/ubuntu.config

@@ -9,6 +9,14 @@ CONFIG_DEBUG_INFO_NONE=y
 # CONFIG_DEBUG_INFO_DWARF4 is not set
 # CONFIG_DEBUG_INFO_DWARF5 is not set
 
+##
+## Enforce xz compression (zstd is unavailable on older distros)
+##
+# CONFIG_MODULE_COMPRESS_NONE is not set
+# CONFIG_MODULE_COMPRESS_GZIP is not set
+CONFIG_MODULE_COMPRESS_XZ=y
+# CONFIG_MODULE_COMPRESS_ZSTD is not set
+
 ##
 ## Do not build samples and tests
 ##