tools/xenstore: Consolidate PRINTF_ATTRIBUTE() in utils.h

tools/xenstore: Consolidate PRINTF_ATTRIBUTE() in utils.h

At the moment PRINTF_ATTRIBUTE() is defined in two places:
    - tdb.h: Defined as a NOP
    - talloc.h: Defined as a NOP for GCC older than 3.0 otherwise will
    add the attribute to check the printf format

Xen requires to build with minimum GCC 4.1 and we want to check the
printf format for all the printf-like functions.

Only implement PRINTF_ATTRIBUTE() once in utils.h and drop the
conditional check for GCC < 3.0.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>