[libatomic, testsuite] Add missing include in atomic-generic.c

[libatomic, testsuite] Add missing include in atomic-generic.c

When compiling atomic-generic.c from the libatomic testsuite, we run into:
...
$ gcc src/libatomic/testsuite/libatomic.c/atomic-generic.c -latomic
src/libatomic/testsuite/libatomic.c/atomic-generic.c: In function ‘main’:
src/libatomic/testsuite/libatomic.c/atomic-generic.c:31:7: warning: \
  implicit declaration of function ‘memcmp’ [-Wimplicit-function-declaration]
   if (memcmp (&a, &zero, size))
       ^~~~~~
...

Fix this by adding the missing string.h include.

Tested on x86_64.

libatomic/ChangeLog:

	* testsuite/libatomic.c/atomic-generic.c: Include string.h.