README: add libc development packages in requirements

Problem: It has been shown that when libc6-dev is missing on a
host system, the build fails in a confusing way:

 ./ccan/endian/endian.h:112:2: error: #error "Unknown endian"
  112 | #error "Unknown endian"
      |  ^~~~~

Explicitly list that glibc development packages are requirements
to build flux-core to avoid this (and probably other) errors.

Fixes flux-framework#4002

README.md

@@ -44,6 +44,7 @@ automake          | automake          |                   |
 libtool           | libtool           |                   |
 make              | make              |                   |
 pkgconfig         | pkg-config        |                   |
+glibc-devel       | libc6-dev         |                   |
 zeromq4-devel     | libzmq3-dev       | >= 4.0.4          |
 czmq-devel        | libczmq-dev       | >= 3.0.1          |
 jansson-devel     | libjansson-dev    | >= 2.6            |
@@ -77,12 +78,12 @@ jq                | jq                |
 
 ##### Installing RedHat/CentOS Packages
 ```
-yum install autoconf automake libtool make pkgconfig zeromq4-devel czmq-devel libuuid-devel jansson-devel lz4-devel hwloc-devel sqlite-devel lua lua-devel lua-posix python36-devel python36-cffi python36-yaml python36-jsonschema python3-sphinx aspell aspell-en valgrind-devel mpich-devel jq
+yum install autoconf automake libtool make pkgconfig glibc-devel zeromq4-devel czmq-devel libuuid-devel jansson-devel lz4-devel hwloc-devel sqlite-devel lua lua-devel lua-posix python36-devel python36-cffi python36-yaml python36-jsonschema python3-sphinx aspell aspell-en valgrind-devel mpich-devel jq
 ```
 
 ##### Installing Ubuntu Packages
 ```
-apt install autoconf automake libtool make pkg-config libzmq3-dev libczmq-dev uuid-dev libjansson-dev liblz4-dev libhwloc-dev libsqlite3-dev lua5.1 liblua5.1-dev lua-posix python3-dev python3-cffi python3-yaml python3-jsonschema python3-sphinx aspell aspell-en valgrind libmpich-dev jq
+apt install autoconf automake libtool make pkg-config libc6-dev libzmq3-dev libczmq-dev uuid-dev libjansson-dev liblz4-dev libhwloc-dev libsqlite3-dev lua5.1 liblua5.1-dev lua-posix python3-dev python3-cffi python3-yaml python3-jsonschema python3-sphinx aspell aspell-en valgrind libmpich-dev jq
 ```
 
 ##### Building from Source