build: fix directory creation for core headers, introduced by d471a219.
diff --git a/include/Makefile.am b/include/Makefile.am
index 61ab67d..60b9ea9 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -100,5 +100,5 @@
 noinst_HEADERS = osmocom/core/timer_compat.h
 
 osmocom/core/crc%gen.h: osmocom/core/crcXXgen.h.tpl
-	$(AM_V_GEN)$(MKDIR_P) $(notdir $@)
+	$(AM_V_GEN)$(MKDIR_P) $(dir $@)
 	$(AM_V_GEN)sed -e's/XX/$*/g' $< > $@