tests: fix libhostname visibility

I noticed that a patched cmake build would pass tests with a fake local
hostname, but the autotools build skips them:

    got unexpected host name back, LD_PRELOAD failed

It turns out that -fvisibility=hidden hides the symbol, and since the
tests are not part of libcurl, it fails too. Just remove the LIBCURL
guard.

Broken since cURL 7.30 (commit 83a42ee2,
"curl.h: stricter CURL_EXTERN linkage decorations logic").

Signed-off-by: Peter Wu <peter@lekensteyn.nl>