The declaration of function error_set() should use macro GCC_FMT_ATTR
instead of gcc's format printf attribute.

For w32/w64, both declarations are different and GCC_FMT_ATTR is needed.
Compilation for w64 even failed with the original code because mingw64
defines a macro for printf.

GCC_FMT_ATTR requires qemu-common.h, so add it in error.c
(it's also included by error_int.h but too late).

Remove assert.h which is included by qemu-common.h.

Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>