dispatch: Add standard boilerplate and GL_APIENTRY to es1_conversion.h.

dispatch: Add standard boilerplate and GL_APIENTRY to es1_conversion.h.

This patch adds the usual boilerplate (copyright notice and guards
against redundant inclusion) to es1_conversion.h.  It also moves the
definition of GL_APIENTRY from es1_conversion.c.

This allows es1_conversion.h to be safely included from other .c files.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
v2: Use copyright notice from src/mesa/main/es_generator.py (the
script that used to generate this file).