glsl: Clean-up spurious error message on bad structure definitions

glsl: Clean-up spurious error message on bad structure definitions

Previously a shader like

int X;
struct X { int i; };

void main() { gl_Position = vec4(0.0); }

would generate two error message:

0:2(19): error: struct `X' previously defined
0:2(20): error: incomplete declaration

The first one is the real error, and the second is spurious.

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