[sdk][build] Only reference header files once in build rule

Use `public` directly rather than setting its value to `sources`. This
is correct and avoids an error about a file being defined by multiple
atoms (though it is just this one) during IDK generation when building
//sdk/firmware.

Change-Id: I0e5722952f6c8a98e714d1fc3d61bb5df59bc25e
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/vboot_reference/+/1357835
Reviewed-by: Yecheng Zhao <zyecheng@google.com>

firmware/BUILD.gn

diff --git a/firmware/BUILD.gn b/firmware/BUILD.gn
index 4c760dc..cf2882b 100644
--- a/firmware/BUILD.gn
+++ b/firmware/BUILD.gn
@@ -15,7 +15,7 @@
 
   # TODO(https://fxbug.dev/331991540): Determine an appropriate solution for building firmware SDK.
   category = "firmware_sdk"
-  sources = [
+  public = [
     "include/bmpblk_header.h",
     "include/gbb_access.h",
     "include/gpt.h",
@@ -39,6 +39,5 @@
     public_deps = [ "//zircon/kernel/lib/libc" ]
   }
 
-  public = sources
   public_configs = [ ":includes" ]
 }