Fix curl free() ambiguity - fixes windows issues

curl_free() is to be used for data allocated by
curl_easy_escape() or curl_easy_unescape() in libcurl.

This is unrelated to realloc/free in WriteMemoryCallback().

gtk2_ardour/ardour_http.cc

@@ -180,7 +180,7 @@ HttpGet::~HttpGet ()
 		curl_easy_cleanup (_curl);
 	}
 	if (!persist) {
-		free (mem.data);
+		::free (mem.data);
 	}
 }
 
@@ -212,7 +212,7 @@ HttpGet::get (const char* url, bool with_error_logging)
 	}
 
 	if (!persist) {
-		free (mem.data);
+		::free (mem.data);
 	} // otherwise caller is expected to have free()d or re-used it.
 
 	error_buffer[0] = 0;

gtk2_ardour/ardour_http.h

@@ -62,6 +62,8 @@ class HttpGet {
 		return curl_easy_unescape (_curl, s, l, o);
 	}
 
+	/* this is only to be used for data returned by from
+	 * \ref escape() and \ref unescape() */
 	void free (void *p) const {
 		curl_free (p);
 	}