build: use the standard AM_V_GEN definition for output.

This actually allows for the command to be printed if using `make V=1`

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
diff --git a/include/Makefile.am b/include/Makefile.am
index 1eae2dd..dec8eac 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -100,5 +100,4 @@
 
 osmocom/core/crc%gen.h: osmocom/core/crcXXgen.h.tpl
 	$(AM_V_GEN)$(MKDIR_P) $(notdir $@)
-	@echo "  SED    $< -> $@"
-	@sed -e's/XX/$*/g' $< > $@
+	$(AM_V_GEN)sed -e's/XX/$*/g' $< > $@