fp-image: Remove config.h inclusion in fpi-header

And also avoid defining a function we expose depending on it's presency.

libfprint/fpi-image.h

@@ -20,7 +20,6 @@
 
 #pragma once
 
-#include <config.h>
 #include "fp-image.h"
 
 /**
@@ -77,8 +76,6 @@ gint fpi_mean_sq_diff_norm (const guint8 *buf1,
                             const guint8 *buf2,
                             gint          size);
 
-#if HAVE_PIXMAN
 FpImage *fpi_image_resize (FpImage *orig,
                            guint    w_factor,
                            guint    h_factor);
-#endif