mesa3d-etna_viv: no longer need MESA_EGL_NO_X11_HEADERS

Override eglplatform.h so that defining MESA_EGL_NO_X11_HEADERS no
longer needs to be defined when compiling mesa itself or software using
it.
Merge pull request #8 from laanwj/opendingux-2012.11

mesa3d-etna_viv: no longer need MESA_EGL_NO_X11_HEADERS