sysklogd: override SKFLAGS with TARGET_CFLAGS

TARGET_CFLAGS is not taken into account since SKFLAGS is used as CFLAGS.
We need to override SKFLAGS with TARGET_CFLAGS but keep -DSYSV which was
present in SKFLAGS.

Improve the previous fix introduced by
8e3a5c13.

Fixes:
http://autobuild.buildroot.net/results/2fb/2fb94ecfdc32761a09da35e6fcbdd512847ec911



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Reported-by: Peter Korsgaard <peter@korsgaard.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>