libs/guest: rework CFLAGS

libs/guest: rework CFLAGS

Remove '-Werror -Wmissing-progress -I./include $(CFLAGS_xeninclude)',
those flags are already added via "libs.mk".

Flag "-I." isn't needed, we just need to fix the #include of
"xg_core.h" as this header isn't expected to be installed.

Make use of "-iquote" instead of '-I' for double-quote included
headers.

Also, regroup the CFLAGS into the same place in the makefile.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>