mapi/glapi: Fix dll linkage of GLES1 symbols.

mapi/glapi: Fix dll linkage of GLES1 symbols.

This fixes several MSVC warnings like:

  warning C4273: 'glClearColorx' : inconsistent dll linkage

In fact, we should avoid using `declspec(dllexport)` altogether, and use
exclusively the .DEF instead, which gives more precise control of which
symbols must be exported, but all the public GL/GLES headers practically
force us to pick between `declspec(dllexport)` or
`declspec(dllimport)`.

Cc: "10.4" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>