libpng - enable neon support

Neon support failed on aosp_arm-eng because that
build didn't have NEON support but yet NEON flags
were applied unconditionally.

This change conditions NEON flags on ARCH_ARM_HAVE_NEON.

Change-Id: Id8dbf98e6f2bdd85857ac0836fde1b4c6ff37cc5
1 file changed