tgsi: include struct definitions for tgsi_build declarations

tgsi: include struct definitions for tgsi_build declarations

Many of the functions declared in tgsi_build.h return structs (not struct
pointers). Therefore the full struct definitions are needed to avoid
warnings or errors:

In file included from src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp:23:
external/mesa3d/src/gallium/auxiliary/tgsi/tgsi_build.h:47:1: error: 'tgsi_build_header' has C-linkage specified, but returns incomplete type 'struct tgsi_header' which could be incompatible with C [-Werror,-Wreturn-type-c-linkage]

This error shows up on Android builds using clang and -Werror.

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Rob Herring <robh@kernel.org>