Use -Werror in external/giflib

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I0c45cf47cefc03064b208be4b05e72c714568019
diff --git a/Android.bp b/Android.bp
index 9bfb163..35705d9 100644
--- a/Android.bp
+++ b/Android.bp
@@ -14,6 +14,7 @@
     ],
 
     cflags: [
+        "-Werror",
         "-Wno-format",
         "-Wno-sign-compare",
         "-Wno-unused-parameter",