Cleanup: make remaining gpu headers work in C++

source/blender/gpu/GPU_immediate.h

@@ -33,6 +33,10 @@
 #include "GPU_immediate_util.h"
 #include "GPU_shader.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** Returns a cleared vertex format, ready for #add_attr. */
 GPUVertFormat *immVertexFormat(void);
 
@@ -146,4 +150,8 @@ void immActivate(void);
 void immDeactivate(void);
 void immDestroy(void);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __GPU_IMMEDIATE_H__ */