glapi: Add positional argument specifier.

glapi: Add positional argument specifier.

Fix build error introduced with commit 1c5a57a "glapi/es3.1: Add support
for GLES versions > 3.0" with Python < 2.7.

  File "src/mapi/glapi/gen/gl_genexec.py", line 230, in <module>
    printer.Print(api)
  File "src/mapi/glapi/gen/gl_XML.py", line 120, in Print
    self.printBody(api)
  File "src/mapi/glapi/gen/gl_genexec.py", line 187, in printBody
    condition_parts.append('(ctx->API == API_OPENGLES2 && ctx->Version >= {})'.format(int(f.api_map['es2'] * 10)))
ValueError: zero length field name in format

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>