Fix incorrect -I option in sys/conf/Makefile.arm

In commit 8e53cd7099436 the intent was to add sys/dts/include to the
compiler include path, but this was spelled incorrectly, leading to an
error with clang 19:

cc: error: no such include directory: '$/dts/include' [-Werror,-Wmissing-include-dirs]

Use the spelling -I$S/dts/include instead.

MFC after: 3 days