TT#76711 add libdl linkage to testing preload lib

Change-Id: I647182dd4abc44ef1f7956df419313215ab82a0f

t/Makefile

@@ -138,7 +138,7 @@ payload-tracker-test: payload-tracker-test.o $(COMMONOBJS) ssrc.o aux.o auxlib.o
 const_str_hash-test.strhash: const_str_hash-test.strhash.o $(COMMONOBJS)
 
 tests-preload.so:	tests-preload.c
-	$(CC) -g -D_GNU_SOURCE -std=c99 -o $@ -Wall -shared -fPIC $<
+	$(CC) -g -D_GNU_SOURCE -std=c99 -o $@ -Wall -shared -fPIC $< -ldl
 
 ifeq ($(with_transcoding),yes)
 .depend: dtmf_rx_fillin.h