This fixes a problem with the move of libc_hidden_proto to string.h.
The obsolete functions bcopy, index, etc. are not supposed to be used within
uClibc itself. Hence, there is no libc_hidden_def for them, but the previous
patch did not just move libc_hidden_protos, it also added new ones for the
legacy functions. As a result, programs which use these functions can no
longer link with uClibc.
This fixes it by removing the unnecessary libc_hidden_protos. I've also
removed all inclusions of <strings.h> from uClibc source files: since we
define _GNU_SOURCE, it is sufficient to include <string.h>. We then do not
need to duplicate the libc_hidden_proto block in <strings.h>.