configure.ac: fix static linking with id3tag

Put back -lz (i.e. ZLIB_LIBS) when searching for id3tag which was
wrongly removed by commit 6ff0e9322f9891f5a6ac6c9b3bceffbfca16bec3. This
will fix the following static build failure:

configure:15290: checking for id3_file_open in -lid3tag
configure:15315: /home/buildroot/autobuild/instance-0/output-1/host/bin/arm-linux-gcc -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -O2 -static -Wall -Wmissing-prototypes -Wstrict-prototypes -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -static -Wl,--as-needed conftest.c -lid3tag >&5
conftest.c:60:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
60 | char id3_file_open ();
| ^~~~
conftest.c:62:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
62 | main ()
| ^~~~
/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/9.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/buildroot/autobuild/instance-0/output-1/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libid3tag.a(util.o): in function id3_util_compress': util.c:(.text+0x1ac): undefined reference tocompress2'
/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/9.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/buildroot/autobuild/instance-0/output-1/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libid3tag.a(util.o): in function id3_util_decompress': util.c:(.text+0x240): undefined reference touncompress'

Fixes:
- http://autobuild.buildroot.org/results/73efdacf237e3d567fa66f3b3f68e624f5e35bc7