Cleanup: Python GPU: Use consistent prefixes for local API

It was not following the own documentation at the top code that mentions
that for local API the prefix is "bpygpu_".

source/blender/python/gpu/gpu_py_matrix.h

@@ -20,4 +20,4 @@
 
 #pragma once
 
-PyObject *BPyInit_gpu_matrix(void);
+PyObject *bpygpu_matrix_init(void);