string.h: fix __fortified_attr_access macro call [BZ #29162]

commit e938c027 "Don't add access size hints to fortifiable functions"
converted a few '__attr_access ((...))' into '__fortified_attr_access (...)'
calls.

But one of conversions had double parentheses of '__fortified_attr_access (...)'.

Noticed as a gnat6 build failure:

    /<<NIX>>-glibc-2.34-210-dev/include/bits/string_fortified.h:110:50: error: macro "__fortified_attr_access" requires 3 arguments, but only 1 given

The change fixes parentheses.

This is seen when using compilers that do not support
__builtin___stpncpy_chk, e.g. gcc older than 4.7, clang older than 2.6
or some compiler not derived from gcc or clang.

Signed-off-by: Sergei Trofimovich <slyich@gmail.com>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit 5a5f94af)