pixman: fix patch for FE_DIVBYZERO detection

pixman: fix patch for FE_DIVBYZERO detection

In commit a9baea4345625f6f00fc59395faec83e08346015 ('pixman: add patch
to fix Microblaze build failure'), a patch is added to the pixman
package to avoid using the FE_DIVBYZERO definition when it is not
available. However, it was using the have_fe_divbyzero variable to
define or not HAVE_FEDIVBYZERO, while the AC_CHECK_DECL autoconf macro
sets the ac_cv_have_decl_FE_DIVBYZERO variable.

The end result was that the FE_DIVBYZERO macro was considered as never
being available. This commit fixes that by using the appropriate
variable.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>