[egl] Install hybris_nativebufferext.h with hybris-egl-platform

This allows external applications to build-depend on the EGL platform
headers and use the EGL extension without having to ship the header
file separately.
Merge pull request #166 from thp/nativebufferext-install-header

[egl] Install hybris_nativebufferext.h with hybris-egl-platform