rust: build: Fix grep warning

Fix grep warning during the build, with GNU grep 3.8
with the following command

`grep -v '^\#\|^$$' rust/bindgen_parameters`

I see the following warning

```
grep: warning: stray \ before #

--opaque-type xregs_state
--opaque-type desc_struct
--opaque-type arch_lbr_state
--opaque-type local_apic

--opaque-type x86_msi_data
--opaque-type x86_msi_addr_lo

--opaque-type kunit_try_catch

--opaque-type spinlock

--no-doc-comments
```
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Tested-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -283,7 +283,7 @@ quiet_cmd_bindgen = BINDGEN $@
 		$(bindgen_target_cflags) $(bindgen_target_extra)
 
 $(obj)/bindings/bindings_generated.rs: private bindgen_target_flags = \
-    $(shell grep -v '^\#\|^$$' $(srctree)/$(src)/bindgen_parameters)
+    $(shell grep -v '^#\|^$$' $(srctree)/$(src)/bindgen_parameters)
 $(obj)/bindings/bindings_generated.rs: $(src)/bindings/bindings_helper.h \
     $(src)/bindgen_parameters FORCE
 	$(call if_changed_dep,bindgen)