Fix pub_tool_basics.h build issue with g++ 4.4.7.

Fix pub_tool_basics.h build issue with g++ 4.4.7.

g++ 4.4.7 doesn't accept union field initializers:
In file included from ../../include/pub_tool_vki.h:50,
                 from valgrind_cpp_test.cpp:13:
../../include/vki/vki-linux.h: In function ‘vki_cmsghdr* __vki_cmsg_nxthdr(void*, __vki_kernel_size_t, vki_cmsghdr*)’:
../../include/vki/vki-linux.h:673: error: expected primary-expression before ‘.’ token

Assign value after declaration which works for any g++ version.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16437