Fix the build for NEON in libpixelflinger

ARCH_ARM_HAVE_NEON is only ever defined to true, so test for that.
For the NEON function to be used, the file has to include
machine/cpu-features.h so that __ARM_HAVE_NEON is defined.

Change-Id: I0db196b39c493092415859e009531fcff6fc1e8b
Signed-off-by: Kévin PETIT <kevin.petit@arm.com>