mapi: Move GL_NV_draw_buffers extension to es_EXT.xml

mapi: Move GL_NV_draw_buffers extension to es_EXT.xml

We don't generate public entrypoints for GLES extensions, so move the
GL_NV_draw_buffers definition from ARB_draw_buffers.xml to es_EXT.xml.
When the extension is defined in ARB_draw_buffers.xml, we end up with a
public entry point for it, but no prototype, which gives an error when
compiled with --disable-asm and --disable-shared-glapi.

Instead, just move the GLES extension to es_EXT.xml so this doesn't happen.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>