cmake: fix building with both md4 and md5 in unity mode

[vszakats]

Macro and static function names were colliding between lib/md4.c and lib/md5.c.

Fix it by namespacing these symbols.

Seen with a basic macOS build using these options: -DCMAKE_UNITY_BUILD=ON -DCURL_USE_SECTRANSP=ON

Closes #13737