MAINTAINERS: assign include/os.h

os.h is only used by the sandbox.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 0257526..f752132 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1361,6 +1361,7 @@
 F:	doc/arch/sandbox.rst
 F:	drivers/*/*sandbox*.c
 F:	include/dt-bindings/*/sandbox*.h
+F:	include/os.h
 
 SEAMA
 M:	Linus Walleij <linus.walleij@linaro.org>