ACPI: OSL: add __printf format attribute to acpi_os_vprintf()

With gcc and W=1 option to compile the kernel, warning occurs:

drivers/acpi/osl.c:156:2: error:
function ‘acpi_os_vprintf’ might be a candidate for ‘gnu_printf’
format attribute [-Werror=suggest-attribute=format].

Allow the compiler to recognize and check format strings is safer.
Signed-off-by: Su Hui <suhui@nfschina.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/osl.c

@@ -149,7 +149,7 @@ void acpi_os_printf(const char *fmt, ...)
 }
 EXPORT_SYMBOL(acpi_os_printf);
 
-void acpi_os_vprintf(const char *fmt, va_list args)
+void __printf(1, 0) acpi_os_vprintf(const char *fmt, va_list args)
 {
 	static char buffer[512];