Fix _IO_JUMPS_OFFSET -Wundef warnings

ChangeLog:

2014-03-17  Will Newton  <will.newton@linaro.org>

	* libio/genops.c: Check whether
	_IO_JUMPS_OFFSET is defined with #ifdef rather
	than #if.
	* libio/libioP.h: Likewise.
	* stdio-common/vfprintf.c: Likewise.