Cortex-M: add matmul_to_bmm_pass.py to the passes BUCK srcs (#21042)

### Summary
#21028 added backends/cortex_m/passes/matmul_to_bmm_pass.py and imported
MatmulToBmmPass from cortex_m_pass_manager, but did not add the file to
the cortex_passes python_library srcs list, so the buck build cannot
resolve the import and internal builds and tests break. Add the missing
source; its imports (torch, exir pass_base) are already covered by the
target's deps.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rascani

backends/cortex_m/passes/BUCK

@@ -35,6 +35,7 @@ fbcode_target(_kind = runtime.python_library,
         "cortex_m_pass_manager.py",
         "decompose_hardswish_pass.py",
         "decompose_mean_pass.py",
+        "matmul_to_bmm_pass.py",
         "quantized_clamp_activation_pass.py",
         "scratch_buffer_sizes.py",
     ],