cairo: fix build on Blackfin in test/cairo-test-runner.c

Add a patch for Cairo that ensures <unistd.h> is properly included to
get the prototype of readlink and getppid even on non-MMU platforms.

Fixes:

  http://autobuild.buildroot.net/results/93a97f90345d193b52035b2dc7559a306e11098e/



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