Fix build on Alpine Linux

[1ma]

This cleanup commit broke the build of CLN v25.12 on Alpine Linux:

common/configvar.c:118:9: error: unknown type name 'ssize_t'; did you mean 'size_t'?
  118 |         ssize_t prev = -1;
      |         ^~~~~~~
      |         size_t

This commit restores the original fix for this issue.