bpo-30183: Fixes HP-UX cc compilation error in pytime.c (#1351)

* bpo-30183: Fixes HP-UX cc compilation error in pytime.c

HP-UX does not support the CLOCK_MONOTONIC identifier, and will fail to
compile:

    "Python/pytime.c", line 723: error #2020: identifier
    "CLOCK_MONOTONIC" is undefined
          const clockid_t clk_id = CLOCK_MONOTONIC;

Add a new section for __hpux that calls 'gethrtime()' instead of
'clock_gettime()'.

* bpo-30183: Removes unnecessary return