acpi: Add missing GCC_FMT_ATTR to local function

This fixes these gcc warnings (not enabled in default build):

hw/acpi/aml-build.c:83:5: warning:
 function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
hw/acpi/aml-build.c:88:5: warning:
 function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/acpi/aml-build.c

@@ -68,7 +68,7 @@ build_append_nameseg(GArray *array, const char *seg)
     g_array_append_vals(array, "____", ACPI_NAMESEG_LEN - len);
 }
 
-static void
+static void GCC_FMT_ATTR(2, 0)
 build_append_namestringv(GArray *array, const char *format, va_list ap)
 {
     /* It would be nicer to use g_string_vprintf but it's only there in 2.22 */