zlib: Use relative paths in pkg-config metadata file

zlib: Use relative paths in pkg-config metadata file

The buildroot pkg-config (in staging_dir/host/bin) overrides the prefix
and exec_prefix variables in *.pc files, to supply the correct
(buildroot) paths for callers. If other variables are not defined
relative to prefix and exec_prefix, then the returned values will be
incorrect.

The default zlib.pc file generated by cmake contains absolute paths.
This patches the file to use relative paths (relative to ${prefix} and
${exec_prefix}).

Signed-off-by: Jeffery To <jeffery.to@gmail.com>