i965: Make backend_instruction usable from C.

With a hack to place an exec_node in the struct in C to be at the same
location as the inherited exec_node in C++.

Acked-by: Topi Pohjolainen <topi.pohjolainen@intel.com>

src/mesa/drivers/dri/i965/brw_shader.h

@@ -78,12 +78,10 @@ struct backend_reg
    bool abs;
 };
 
-#ifdef __cplusplus
-
 struct cfg_t;
 
+#ifdef __cplusplus
 struct backend_instruction : public exec_node {
-public:
    bool is_tex() const;
    bool is_math() const;
    bool is_control_flow() const;
@@ -98,7 +96,10 @@ public:
     * optimize these out unless you know what you are doing.
     */
    bool has_side_effects() const;
-
+#else
+struct backend_instruction {
+   struct exec_node link;
+#endif
    /** @{
     * Annotation for the generated IR.  One of the two can be set.
     */
@@ -124,6 +125,8 @@ public:
    bool saturate:1;
 };
 
+#ifdef __cplusplus
+
 enum instruction_scheduler_mode {
    SCHEDULE_PRE,
    SCHEDULE_PRE_NON_LIFO,