security/wpa_supplicant: Fix build on FreeBSD 11

On FreeBSD 11, the wpa_supplicant couldn't find the readline headers.
It turns out that Mk/Uses/readline.mk only augments LDFLAGS and CPPFLAGS,
but wpa_supplicant needs -I${LOCALBASE}/include add to CFLAGS.  To fix
FreeBSD build, augment CFLAGS with value of CPPFLAGS in the makefile.

This wasn't an issue on DragonFly.  Apparently wpa_supplicant found the
readline compatibility headers of libedit and used those instead.

reported by:	pkg-fallout


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@371174 35697150-7ecd-e111-bb59-0022644237b5