Use -Werror in external/libhevc

* Keep constant-conversion warnings.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I4b956b73654029be940f911a4741be922fb3559b
diff --git a/Android.bp b/Android.bp
index d1067a6..80332d6 100644
--- a/Android.bp
+++ b/Android.bp
@@ -8,6 +8,11 @@
 
         "-O3",
         "-DANDROID",
+        "-Wall",
+        "-Werror",
+        // common/x86/ihevc_sao_ssse3_intr.c: implicit conversion from
+        // 'int' to 'char' changes value from 128 to -128
+        "-Wno-error=constant-conversion",
     ],
 
     export_include_dirs: [