[bazel] Disable warnings to match BUILD.gn

Update the BUILD.bazel file to disable a few warnings,
matching the content of BUILD.gn. This is a requirement
to port existing default GN config()s to Bazel, which
will make conversion warnings enabled by default for
all Bazel targets.

Bug: 565437401
Change-Id: I065346ba9b80bdcd181cf83b5ace175d7bcff20f
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/zlib/+/1862995
Reviewed-by: Francois Rousseau <frousseau@google.com>
Reviewed-by: David Fang <fangism@google.com>

BUILD.bazel

diff --git a/BUILD.bazel b/BUILD.bazel
index c5cfca6..a984146 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -78,6 +78,9 @@
             "-Wno-unused-variable",
             "-Wno-implicit-function-declaration",
             "-Wno-strict-prototypes",
+            "-Wno-conversion",
+            "-Wno-implicit-fallthrough",
+            "-Wno-call-graph-section-no-prototype",
         ],
     }),
     visibility = ["//visibility:public"],