bitbang_spi: Move API into its own header file

We're going to extent it for multi-i/o.

Change-Id: Ifead97d7a8f848b82a4d21c557f5d364066d5d6a
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/81
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/rayer_spi.c b/rayer_spi.c
index f8bac35..30577ac 100644
--- a/rayer_spi.c
+++ b/rayer_spi.c
@@ -28,6 +28,7 @@
 #include <string.h>
 #include "flash.h"
 #include "programmer.h"
+#include "bitbang_spi.h"
 #include "hwaccess_x86_io.h"
 
 /* We have two sets of pins, out and in. The numbers for both sets are