C++17 compatibility.

  libnativehelper/JNIHelp.cpp:365:27: error: function
  'impl::realJniStrError<char *(*)(int, char *, unsigned long) noexcept>'
  has internal linkage but is not defined [-Werror,-Wundefined-internal]

Bug: http://b/111067277
Test: builds

Change-Id: I66d40e96899d830501885a49c17a5426bac174f6