Set 'incbin' target inputs

'incbin' targets would not recompile when the input files changed.
This was because they were not included in the 'inputs' list of the
target.

Change-Id: I1da023d716e6d011b2d549461afe25d57a507d89
diff --git a/build/image/image.gni b/build/image/image.gni
index e812fe6..f116cb6 100644
--- a/build/image/image.gni
+++ b/build/image/image.gni
@@ -183,14 +183,13 @@
                              "deps",
                            ])
 
-    inc_files = invoker.sources
-
     sources = [
       "//build/image/incbin.S",
     ]
+    inputs = invoker.sources
     defines = [
       "SECTION_NAME=" + invoker.section,
-      "FILE_PATH=\"" + rebase_path(inc_files[0]) + "\"",
+      "FILE_PATH=\"" + rebase_path(inputs[0]) + "\"",
     ]
   }
 }