readstabs.c: Don't include a.out.h, just define the stabs symbol nlist entry.

readstabs.c tries to include a.out.h to get the stabs symbol list entry
definition. STABS isn't specifically tied to the a.out format though.
The symbol entry structure just happens to be defined in the a.out.h
header. The header isn't really standard though. It might be provided
by glibc or the kernel in different locations. And not all arches support
the a.out format so the header might not even exist. Just define the
needed nlist struct entry directly in readstabs.c for VGO_linux. All
arches in glibc and the kernel use the same one anyway.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14484 a5019735-40e9-0310-863c-91ae7b9d1cf9