OpenGL: let eglext_angle.h include eglext.h for platform

EGLStreamKHR and EGLDeviceEXT used by eglext_angle.h is undefined
without eglext.h. From the description of eglext_angle.h, it's just
for patching eglext.h and should never be explictly included. However,
eglext.h is generated from EGL spec. So we'd better not manually modify
eglext.h to include eglext_angle.h at the bottom. This change just let
eglext_angle.h include eglext.h.

Bug: 158235687
Test: build, flash and boot
Change-Id: I02f992d880cce41b3ddc70fb5a28a87a64f05ec7