gdb: fix array-view-selftests.c build with g++ 4.8

gdb: fix array-view-selftests.c build with g++ 4.8

When building with g++ 4.8, I get:

    CXX    unittests/array-view-selftests.o
  /home/smarchi/src/binutils-gdb/gdb/unittests/array-view-selftests.c:123:42: error: expected 'class' before 'Container'
   template<template<typename ...> typename Container>
					    ^

I am no C++ template expert, but it looks like if I change "typename" for
"class", as the compiler kind of suggests, the code compiles.

Change-Id: I9c3edd29fb2b190069f0ce0dbf3bc3604d175f48