Reversed the logic to only include the <sys/select.h> header on systems

known to really NEED it as another system that doesn't have it came up:
very old Linux libc5-based systems (as addition to all HPUX versions).

The only known system at this point is AIX.

include/curl/multi.h

@@ -54,10 +54,14 @@
 #if defined(WIN32) && !defined(__GNUC__) || defined(__MINGW32__)
 #include <winsock.h>
 #else
-#ifndef __hpux
-/* HP-UX systems version 9, 10 and 11 lack this header */
+
+#ifdef _AIX
+/* HP-UX systems version 9, 10 and 11 lack sys/select.h and so does oldish
+   libc5-based Linux systems. Only include it on system that are known to
+   require it! */
 #include <sys/select.h>
 #endif
+
 #include <sys/socket.h>
 #include <sys/time.h>
 #include <sys/types.h>