tests: Fix potential link errors. * modules/mbiter-bench-tests (Makefile.am): Link the benchmark program with $(LIBICONV). * modules/mbiterf-bench-tests (Makefile.am): Likewise. * modules/mbuiter-bench-tests (Makefile.am): Likewise. * modules/mbuiterf-bench-tests (Makefile.am): Likewise. * modules/mcel-bench-tests (Makefile.am): Likewise. * modules/mbswidth-bench-tests (Makefile.am): Likewise.