package infra: remove empty TARGET_CPPFLAGS def

It's unneeded, just fills the next match (LARGEFILE) with a leading
space. Fixes
http://autobuild.buildroot.net/results/ad5772d19036a0c5564228ac7aba953d35a3cf85/build-end.log


since SCons can't handle leading whitespace in LDFLAGS gracefully.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>