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/linux_gpio2_spi.c b/linux_gpio2_spi.c
index ef162b8..24cf33d 100644
--- a/linux_gpio2_spi.c
+++ b/linux_gpio2_spi.c
@@ -23,6 +23,7 @@
 #include <errno.h>
 #include <gpiod.h>
 #include "programmer.h"
+#include "bitbang_spi.h"
 #include "spi.h"
 #include "flash.h"