Add '-ansi' to DTS_CPPFLAGS to avoid unwanted expansion of dts content
that matches some predefined macros.
Example: A number of PowerPC related *.dts files in the kernel define a
property named 'linux,network-index' which (w/o '-ansi') is expanded to
'1,network-index' by the preprocessor because of '#define linux 1.'
Signed-off-by: Horst Kronstorfer <firstname.lastname@example.org>