fbgrab: fix static linking

Fixes:

  http://autobuild.buildroot.org/results/9e0/9e079632a6c0556c96ec8ed7564286aa84044bf7/
  http://autobuild.buildroot.org/results/401/401105b5e120176e78c2e5fcabe54ab0bde1bf40/


  and more

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