"-I." is there, so instead of "../lib/ipv4.h" prefer <lib/ipv4.h>.

I thought moving ipv4.h to include, but then the name is possibly ambiguous
with other files in the -I search path. Thus I wanted to move
it from lib/ipv4.h to include/wrc-ipv4.h but that would require several
files to be changed.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>