fix: disable `-Wsign-compare`

The `-Wextra` flag was enabled in the parent `BUILD.gn`, which causes
`dtc` to inherit the `-Wextra` flag and fail to compile with
`-Wsign-compare` errors.

Change-Id: I3ecd8871bda1620d5bf741814f4cf342ac22ed36
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/BUILD.gn b/BUILD.gn
index f55560c..17ed5e9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -13,6 +13,7 @@
   cflags = [
     "-Wno-zero-length-array",
     "-Wno-extra-semi",
+    "-Wno-sign-compare",
   ]
 }