MAINTAINERS: add regulator.rs to the regulator API entry

Add this file to the regulator API entry as requested by Mark Brown.

Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20250714-topics-tyr-regulator2-v8-2-c7ab3955d524@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: danielalmeida-collabora

MAINTAINERS

@@ -26561,6 +26561,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator.git
 F:	Documentation/devicetree/bindings/regulator/
 F:	Documentation/power/regulator/
 F:	drivers/regulator/
+F:	rust/kernel/regulator.rs
 F:	include/dt-bindings/regulator/
 F:	include/linux/regulator/
 K:	regulator_get_optional