package/irrlicht: disable bundled libpng NEON support

Irrlicht fail to detect properly the NEON support on aarch64 or ARM with NEON FPU support.
While linking an application with libIrrlicht.so, we get an undefined reference to
png_init_filter_functions_neon.
Some files are missing in the libpng bundled in Irrlicht, in particular arm/arm_init.c [1],
so disable NEON support completely.

This can be reproduced by building minetest using this defconfig for aarch64:
BR2_aarch64=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_PACKAGE_MINETEST=y
BR2_PACKAGE_MINETEST_CLIENT=y
BR2_PACKAGE_MINETEST_SERVER=y
BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_SWRAST=y
BR2_PACKAGE_MESA3D_OPENGL_GLX=y
BR2_PACKAGE_XORG7=y

Or for ARM with NEON FPU support:
BR2_arm=y
BR2_cortex_a15=y
BR2_ARM_FPU_NEON=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_PACKAGE_MINETEST=y
BR2_PACKAGE_MINETEST_CLIENT=y
BR2_PACKAGE_MINETEST_SERVER=y
BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_SWRAST=y
BR2_PACKAGE_MESA3D_OPENGL_GLX=y
BR2_PACKAGE_XORG7=y

[1] https://github.com/glennrp/libpng/tree/v1.6.37/arm



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>