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 83a42ee,
"curl.h: stricter CURL_EXTERN linkage decorations logic").

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

tests/libtest/sethostname.h

@@ -23,12 +23,8 @@
 #ifdef CURL_STATICLIB
 #  define LIBHOSTNAME_EXTERN
 #elif defined(WIN32) || defined(__SYMBIAN32__)
-#  if defined(BUILDING_LIBCURL)
-#    define LIBHOSTNAME_EXTERN  __declspec(dllexport)
-#  else
-#    define LIBHOSTNAME_EXTERN  __declspec(dllimport)
-#  endif
-#elif defined(BUILDING_LIBCURL) && defined(CURL_HIDDEN_SYMBOLS)
+#  define LIBHOSTNAME_EXTERN  __declspec(dllexport)
+#elif defined(CURL_HIDDEN_SYMBOLS)
 #  define LIBHOSTNAME_EXTERN CURL_EXTERN_SYMBOL
 #else
 #  define LIBHOSTNAME_EXTERN