Use gethrtime() on HP-UX for timers.

HP-UX does not have clock_gettime(CLOCK_MONOTONIC), instead use
platform-specific gethrtime() function.

Thanks to zhongk for this fix.
Correct #include <sys/uio.h> location.

sys/uio.h should be included in amqp_private.h and not amqp.h, it also should
not be protected by a test for the GCC compiler.

This should fix the issue with compiling rabbitmq-c on HP-UX using aCC.

This fixes #183.