sysstat: fix passing CFLAGS to make

SYSSTAT_MAKE_OPT is a string used to pass options to make, '+=' operator
doesn't have any special meaning inside it, so CFLAGS+=... is passed to
shell, overwriting previous CFLAGS value.

Replace CFLAGS+="..." with CFLAGS="$(TARGET_CFLAGS) ...".

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>