boot: Add some missing unused arguments

In MCUBOOT_RAM_LOAD mode, bootutil_img_hash has some arguments that end
up being unused, which creates warnings.  Add these to the list of
variables intentionally unused.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/boot/bootutil/src/image_validate.c b/boot/bootutil/src/image_validate.c
index f066add..2d368fd 100644
--- a/boot/bootutil/src/image_validate.c
+++ b/boot/bootutil/src/image_validate.c
@@ -85,6 +85,9 @@
     (void)blk_sz;
     (void)off;
     (void)rc;
+    (void)fap;
+    (void)tmp_buf;
+    (void)tmp_buf_sz;
 #endif
 #endif