gallium/tgsi: Define the TGSI_BUFFER texture target.

This texture type was already referred to by the documentation but it
was never defined.  Define it as 0 to match the pipe_texture_target
enumeration values.

src/gallium/include/pipe/p_shader_tokens.h

@@ -466,7 +466,7 @@ struct tgsi_instruction_label
    unsigned Padding  : 8;
 };
 
-#define TGSI_TEXTURE_UNKNOWN        0
+#define TGSI_BUFFER                 0
 #define TGSI_TEXTURE_1D             1
 #define TGSI_TEXTURE_2D             2
 #define TGSI_TEXTURE_3D             3