Squelch two JNI build warnings

[RyanTheOptimist]

I currently see 3 build warnings when I try to build Envoy Mobile.

INFO: From Compiling library/common/jni/jni_interface.cc:
library/common/jni/jni_interface.cc:1195:13: warning: unused function 'jvm_verify_x509_cert_chain' [-Wunused-function]
static void jvm_verify_x509_cert_chain(const std::vector<std::string>& cert_chain,
            ^
1 warning generated.
INFO: From Compiling library/common/jni/jni_utility.cc:
In file included from library/common/jni/jni_utility.cc:6:
In file included from external/envoy/source/common/common/assert.h:5:
In file included from external/envoy/source/common/common/logger.h:13:
external/envoy/source/common/common/fine_grain_logger.h:20:1: warning: inline variables are a C++17 extension [-Wc++17-extensions]
inline const char* kDefaultFineGrainLogFormat = "[%Y-%m-%d %T.%e][%t][%l] [%g:%#] %v";
^
library/common/jni/jni_utility.cc:28:32: warning: ISO C++11 does not allow conversion from string literal to 'char *' [-Wwritable-strings]
    static char* thread_name = "EnvoyMain";
                               ^
2 warnings generated.

This PR squelches the two of them that are in Envoy Mobile code.

Signed-off-by: Ryan Hamilton rch@google.com

Risk Level: Low
Testing: Builds
Docs Changes: N/A
Release Notes: N/A

[snowp]

Nice!