gluon.mk: print source file name in GluonSrcDiet

luasrcdiet will not print the name of its input file when an error
occurs. To facilitate debugging, echo the name before calling it, so it
is visible with V=s or BUILD_LOG=1.

package/gluon.mk

@@ -67,6 +67,7 @@ define GluonSrcDiet
   ifdef CONFIG_GLUON_MINIFY
 	# Use cp + rm instead of mv to preserve destination permissions
 	set -e; $(FIND) $(2) -type f | while read src; do \
+		echo "Minifying $$$$src..."; \
 		luasrcdiet --noopt-binequiv -o "$$$$src.tmp" "$$$$src"; \
 		cp "$$$$src.tmp" "$$$$src"; \
 		rm "$$$$src.tmp"; \