[C++] Use lz4::lz4 for lz4's CMake target name

[asfimport]

Because upstream uses lz4::lz4 not LZ4::lz4.

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

Related issues:

• [Packaging] Add support for Coan C/C++ package manager (relates to)

PRs and other links:

• GitHub Pull Request #13193

_{Note: This issue was originally created as ARROW-16614. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 13193
#13193