fix spelling of HAVE_LIBPYTHON2_4 in py-value.c

Ulrich pointed out that an earlier patch had misspelled
HAVE_LIBPYTHON2_4, adding an extra "_".  This caused a build failure.
This patch fixes the bug.

2016-05-25  Tom Tromey  <tom@tromey.com>

	* python/py-value.c (value_object_as_number): Use correct spelling
	of HAVE_LIBPYTHON2_4.