Suppress -Wstring-plus-int warning

clang-r353983 emits the following new warning:

  external/lua/src/lundump.c:237:33: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
    checkliteral(S, LUA_SIGNATURE + 1, "not a");  /* 1st char already checked */
                    ~~~~~~~~~~~~~~^~~
  external/lua/src/lundump.c:237:33: note: use array indexing to silence this warning
    checkliteral(S, LUA_SIGNATURE + 1, "not a");  /* 1st char already checked */
                                  ^
                    &             [  ]

Test: m checkbuild
Bug: 128878287
Change-Id: If5eebce35908b2745d27cab6ad31356d488cccf1
diff --git a/Android.bp b/Android.bp
index 3f6b42f..f97bb44 100644
--- a/Android.bp
+++ b/Android.bp
@@ -19,6 +19,7 @@
         "-Werror",
         "-Wall",
         "-Wno-deprecated-declarations",
+        "-Wno-string-plus-int",
         "-O2",
         "-DLUA_USE_LINUX",
         "-DLUA_COMPAT_5_2",