tools build: Fix the zstd test in the test-all.c common case feature test

tools build: Fix the zstd test in the test-all.c common case feature test

We were renanimg 'main' to 'main_zstd' but then using 'main_libzstd();'
in the main() for test-all.c, causing this:

  $ cat /tmp/build/perf/feature/test-all.make.output
  test-all.c: In function ‘main’:
  test-all.c:236:2: error: implicit declaration of function ‘main_test_libzstd’; did you mean ‘main_test_zstd’? [-Werror=implicit-function-declaration]
    main_test_libzstd();
    ^~~~~~~~~~~~~~~~~
    main_test_zstd
  cc1: all warnings being treated as errors
  $

I.e. what was supposed to be the fast path feature test was _always_
failing, duh, fix it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Fixes: 3b1c5d965971 ("tools build: Implement libzstd feature check, LIBZSTD_DIR and NO_LIBZSTD defines")
Link: https://lkml.kernel.org/n/tip-ma4abk0utroiw4mwpmvnjlru@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>