mesa: Add a flag for shader programs to allow SSO linkage in GLES2.

mesa: Add a flag for shader programs to allow SSO linkage in GLES2.

On converting fixed function programs to generate GLSL, the linker
became cranky that we were trying to make something that wasn't a
linked vertex+fragment program.  Given that the Mesa GLES2 drivers
also support desktop GL with EXT_sso, just telling the linker to shut
up seems like the easiest solution.