glsl: Fix clang mismatched-tags warnings with glsl_type.

glsl: Fix clang mismatched-tags warnings with glsl_type.

Fix clang mismatched-tags warnings introduced with commit
4f5445a45d3ed02e00a061b10c943c0b079c6020.

./glsl_symbol_table.h:37:1: warning: class 'glsl_type' was previously declared as a struct [-Wmismatched-tags]
class glsl_type;
^
./glsl_types.h:86:8: note: previous use is here
struct glsl_type {
       ^
./glsl_symbol_table.h:37:1: note: did you mean struct here?
class glsl_type;
^~~~~

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>