make regen: fix: use $(MAKE) instead of make

On FreeBSD, we use gmake, so calling 'make -C ...' won't work here.
We need to invoke $(MAKE) so gmake is used again.

Change-Id: If8d09889db2c101eca66675fff8f820d6d1d7a65
diff --git a/src/Makefile.am b/src/Makefile.am
index 3709cd0..2d078f3 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -58,6 +58,6 @@
 regen: regenerate-from-asn1-source
 
 regenerate-from-asn1-source:
-	make -C hnbap regen
-	make -C ranap regen
-	make -C rua regen
+	$(MAKE) -C hnbap regen
+	$(MAKE) -C ranap regen
+	$(MAKE) -C rua regen