staging: usbip: userspace: move common cflags to configure.ac

There are a standard set of cflags that are used in each makefile so
let's set those in EXTRA_CFLAGS and enforce them for the entire project.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>