Makefile.am: Replace .PHONY asn1tostruct.py rules

asn1tostruct.py generates three files.  Try to teach the makefile that
all three of them depend on the .asn source file to ensure they're
re-built whenever the .asn source file changes.
diff --git a/src/Makefile.am b/src/Makefile.am
index 156baa4..c0a3824 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -12,14 +12,14 @@
 hnbgw_LDADD = $(OSMOCORE_LIBS) $(OSMOVTY_LIBS) $(OSMOGSM_LIBS) $(ASN1C_LIBS) $(COMMON_LDADD) hnbap/libosmo-asn1-hnbap.a rua/libosmo-asn1-rua.a ranap/libosmo-asn1-ranap.a
 
 
-.PHONY: hnbap_encoder.c
+hnbap_encoder.c hnbap_decoder.c hnbap_ies_defs.h: $(ASN1_ROOT)/hnbap/HNBAP-PDU-Contents.asn
 hnbap_encoder.c : $(ASN1_ROOT)/hnbap/HNBAP-PDU-Contents.asn
 	$(ASN1TOSTRUCT) -f $<
 
-.PHONY: rua_encoder.c
+rua_encoder.c rua_decoder.c rua_ies_defs.h: $(ASN1_ROOT)/rua/RUA-PDU-Contents.asn
 rua_encoder.c : $(ASN1_ROOT)/rua/RUA-PDU-Contents.asn
 	$(ASN1TOSTRUCT) -p RUA_ -f $<
 
-.PHONY: ranap_encoder.c
+ranap_encoder.c rua_decoder.c rua_ies_defs.h : $(ASN1_ROOT)/ranap/RANAP-PDU-Contents.asn
 ranap_encoder.c : $(ASN1_ROOT)/ranap/RANAP-PDU-Contents.asn
 	$(ASN1TOSTRUCT) -p RANAP_ -f $<