egl: fix the EGL_MESA_image_dma_buf_export header declarations

egl: fix the EGL_MESA_image_dma_buf_export header declarations

Similar to other EGL extensions - guard the function prototypes by
EGL_EGLEXT_PROTOTYPES as the libEGL library does (should) not provide
the symbols statically.

Instead users should call eglGetProcAddress, which returns the function
pointer. The latter of which was missing the type declaration (typedef).

Cc: Dave Airlie <airlied@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>