cmAffinity: Add include for CPU_ZERO on Alpine Linux

The definition of `CPU_ZERO` requires `<string.h>` to be included
explicitly for `memset`.  Since IWYU does not agree on other Linux
platforms, use a pragma to silence the warning.

Fixes: #19643

Source/cmAffinity.cxx

@@ -12,6 +12,8 @@
 #    define CM_HAVE_CPU_AFFINITY
 #    include <pthread.h>
 #    include <sched.h>
+// On some platforms CPU_ZERO needs memset but sched.h forgets string.h
+#    include <string.h> // IWYU pragma: keep
 #    if defined(__FreeBSD__)
 #      include <pthread_np.h>
 #      include <sys/cpuset.h>