lib/string_helpers: Add missing header files to MAINTAINERS database

The header files string.h and string_helpers.h are missing in
the MAINTAINERS. Add them.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Author: andy-shev

MAINTAINERS

@@ -8671,6 +8671,8 @@ F:	drivers/input/touchscreen/resistive-adc-touch.c
 GENERIC STRING LIBRARY
 R:	Andy Shevchenko <andy@kernel.org>
 S:	Maintained
+F:	include/linux/string.h
+F:	include/linux/string_helpers.h
 F:	lib/string.c
 F:	lib/string_helpers.c
 F:	lib/test_string.c