iconv: Fix "make make-ref"

The purpose of this command is to "refresh" the source reference
files generated with GNU libiconv, located in tools/test/iconv/ref/.
Previously it would generate copies somewhere in OBJDIR, which
we don't use.

Reviewed By:	allanjude
Sponsored by:   Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D31820

tools/test/iconv/Makefile

@@ -39,9 +39,9 @@ make-ref: refgen
 	mkdir -p ref
 .for enc in ${ENCODING}
 	@echo "Generating ${enc} --> UTF-32 ..."
-	-@${REF_FWD} ${enc} >ref/${enc}
+	-@${REF_FWD} ${enc} >${.CURDIR}/ref/${enc}
 	@echo "Generating UTF-32 --> ${enc} ..."
-	-@${REF_REV} ${enc} >ref/${enc}-rev
+	-@${REF_REV} ${enc} >${.CURDIR}/ref/${enc}-rev
 .endfor
 
 check: tablegen