MAINTAINERS: add proc sysctl KUnit test to PROC SYSCTL section

Add entry for the new proc sysctl KUnit test to the PROC SYSCTL section.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>

MAINTAINERS

diff --git a/MAINTAINERS b/MAINTAINERS
index fc2cedbd..03054e4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -12523,6 +12523,7 @@
 F:	fs/proc/proc_sysctl.c
 F:	include/linux/sysctl.h
 F:	kernel/sysctl.c
+F:	kernel/sysctl-test.c
 F:	tools/testing/selftests/sysctl/
 
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