S1AP: Compile with -flink-skeletons flag

This makes debugging and testing a lot more easier as skeleton files are
not copied and you don't need to execute `make regen` after every change
in the skeleton files.

You need to manually remove all skeleton files and run
`make regen-makefile` the first time after checking out this commit

examples/sample.source.S1AP/Makefile

@@ -10,7 +10,7 @@ S1AP-PDU.c: ../sample.makefile.regen ../s1ap-14.4.0.asn1
 	make
 
 regen-makefile:
-	ASN_CMDOPTS="-pdu=all -fcompound-names -findirect-choice -fno-include-deps -gen-PER" \
+	ASN_CMDOPTS="-pdu=all -fcompound-names -findirect-choice -fno-include-deps -flink-skeletons -gen-PER" \
 	ASN_MODULES="../s1ap-14.4.0.asn1" \
 	ASN_PDU=S1AP-PDU \
 	ASN_PROGRAM=s1ap-dump \