fix(base): rename matmul.h to mat_mul.h to match MatMul class name

The codegen script `generate_wrappers.py` uses `_snake_to_pascal()` to
derive the class name from the filename. `matmul` -> `Matmul`, but the
class was renamed to `MatMul` (ONNX convention). Renaming the file to
`mat_mul.h` makes `_snake_to_pascal("mat_mul")` -> `MatMul`, fixing the
`IndexError: list index out of range` build failure.

Author: zhangyue

src/base/matmul.h src/base/mat_mul.hsrc/base/matmul.h renamed to src/base/mat_mul.h

@@ -1,5 +1,5 @@
-#ifndef INFINI_OPS_BASE_MATMUL_H_
-#define INFINI_OPS_BASE_MATMUL_H_
+#ifndef INFINI_OPS_BASE_MAT_MUL_H_
+#define INFINI_OPS_BASE_MAT_MUL_H_
 
 #include "operator.h"
 #include "tensor.h"