build pre-reqs are missing reference to zlib

[Rowdster]

Issue and Steps to Reproduce

In building c-lightning inside a docker bitcoin container, the build fails with the -lz parameter

With the below, build works. Just need to add this to the pre-reqs notes.

sudo apt-get install zlib1g-dev

here's the failure without zlib

root@kylemanna-bitcoind1:/opt/lightningd# make
[ -f external/jsmn/jsmn.c ]
cc -Og ccan/ccan/cdump/tools/cdump-enumstr.o ccan-cdump.o ccan-strmap.o ccan-asort.o ccan-autodata.o ccan-bitmap.o ccan-bitops.o
ccan-breakpoint.o ccan-crc32c.o ccan-crypto-hmac.o ccan-crypto-hkdf.o ccan-crypto-ripemd160.o ccan-crypto-sha256.o ccan-crypto-sha
chain.o ccan-crypto-siphash24.o ccan-err.o ccan-fdpass.o ccan-htable.o ccan-ilog.o ccan-io-io.o ccan-intmap.o ccan-io-poll.o ccan-i
o-fdpass.o ccan-isaac.o ccan-isaac64.o ccan-json_escape.o ccan-json_out.o ccan-list.o ccan-mem.o ccan-membuf.o ccan-noerr.o ccan-op
t-helpers.o ccan-opt-parse.o ccan-opt-usage.o ccan-opt.o ccan-pipecmd.o ccan-ptr_valid.o ccan-rbuf.o ccan-read_write_all.o ccan-str
-base32.o ccan-str-hex.o ccan-str.o ccan-take.o ccan-tal-grab_file.o ccan-tal-link.o ccan-tal-path.o ccan-tal-str.o ccan-tal.o ccan
-time.o ccan-timer.o ccan-utf8.o -Lexternal -lwallycore -lsecp256k1 -ljsmn -lbacktrace -lsodium -L/usr/local/lib -lm -lgmp -lsqlit
e3 -lz -o ccan/ccan/cdump/tools/cdump-enumstr
/usr/bin/ld: cannot find -lz
collect2: error: ld returned 1 exit status
Makefile:398: recipe for target 'ccan/ccan/cdump/tools/cdump-enumstr' failed
make: *** [ccan/ccan/cdump/tools/cdump-enumstr] Error 1