Fix diffutils build breakage with BR2_STRIP_none=y

Look at BR2_STRIP_none to decide whether to strip diffutils binaries.
This avoids bad quoting in diffutils Makefile.in (generated by automake 1.6)
when STRIP is set to:   true -Not_stripping

Signed-off-by: Marc Gauthier <marc@alumni.uwaterloo.ca>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>