package/Makefile.in: CFLAGS/LDFLAGS: don't add -I / -L args for STAGING_DIR

Now that we use sysroot for all toolchains, the explicit -I / -L arguments
in CFLAGS / LDFLAGS aren't needed anymore (And having them makes the build
quite noisy for certain packages as STAGING_DIR/include normally doesn't
exist).

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