libv4l: fixup after CPPFLAGS addition

Fixes http://autobuild.buildroot.net/results/4602f7cf4407a2969a04267bbc5d276b076a7c91



The preload libraries needs to wrap both the lfs and !lfs variants, so
ensure the lfs versions are not transparently used (as they are when
_LARGEFILE_SOURCE / _FILE_OFFSET_BITS=64 are defined).

This used to work by accident before, as our custom CFLAGS (containing those
defines) wasn't used.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>