gdb: fix ia64-tdep.c build with g++ 4.8

gdb: fix ia64-tdep.c build with g++ 4.8

When building with g++ 4.8, I get:

      CXX    ia64-tdep.o
    /home/smarchi/src/binutils-gdb/gdb/ia64-tdep.c:3862:1: error: could not convert '{ia64_allocate_new_rse_frame, ia64_store_argument_in_slot, ia64_set_function_addr}' from '<brace
-enclosed initializer list>' to 'const ia64_infcall_ops'
     };
     ^

This happens since commit 345bd07cce3 ("gdb: fix gdbarch_tdep ODR
violation"), which added default values for ia64_infcall_ops fields.  It
looks like g++ 4.8 doesn't like initializing the ia64_infcall_ops object
using the brace-enclosed initializer list when the ia64_infcall_ops
fields are assigned default values.

Later compilers don't have a problem with that, so I suppose that the
code is correct, but still, change it to make gcc 4.8 happy.  Don't
initialize the fields of ia64_infcall_ops directly, instead
default-initialize ia64_gdbarch_tdep::infcall_ops.

Change-Id: I35e3a61abd7b7bbcafe6cb207078c738c5266d76