Move macros GCC_ATTR and GCC_FMT_ATTR to common header file

By moving the definition of GCC_ATTR and GCC_FMT_ATTR
from audio_int.h to qemu-common.h these macros are
now generally available for further patches which add
the gcc format attribute.

Newer gcc versions support format gnu_printf which is
better suited for use in QEMU than format printf
(QEMU always uses standard format strings (even with mingw32)).

V2: Use correct operator '==' (instead of '=')

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>