time: mark as not available for static-only builds

Due to some code imported from gnulib, the time package doesn't build
in static-only configurations. Indeed the gnulib code redefines the
error() and error_at_line() functions, which are also provided by the
C library. Since fixing the gnulib code is really difficult, let's
just disable this package for static-only builds.

Fixes:

  http://autobuild.buildroot.org/results/a0e64faba69fa86755c693f575fb258a77e4e9d1/



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