Android: Fix build issue without LLVM

Android: Fix build issue without LLVM

Some of the latest changes are causing the following build error on Android:

```
external/mesa3d/src/gallium/auxiliary/nir/nir_to_tgsi_info.c:403:6:
error: redefinition of 'nir_tgsi_scan_shader'
void nir_tgsi_scan_shader(const struct nir_shader *nir,
     ^
external/mesa3d/src/gallium/auxiliary/nir/nir_to_tgsi_info.h:37:20:
note: previous definition is here
static inline void nir_tgsi_scan_shader(const struct nir_shader *nir,
                   ^
```

Include nir_to_tgsi_info.c and nir_to_tgsi_info.h into the build
only if LLVM is enabled.

Signed-off-by: Roman Stratiienko <roman.stratiienko@globallogic.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2978>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2978>