spi: Define _XOPEN_SOURCE to get ffs() from strings.h

Fixes build with Musl libc.

Change-Id: I92b69300a302c4491e234666ac371027571801c9
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/22951
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>

spi.c

@@ -22,6 +22,8 @@
  * Contains the generic SPI framework
  */
 
+#define _XOPEN_SOURCE 700	/* for ffs() */
+
 #include <strings.h>
 #include <string.h>
 #include "flash.h"