Fix compilation if sys/sysctl.h is not present

Glibc 2.32 removes sys/sysctl.h as it is no longer needed. This commit fixes
the compilation with newer versions of Glibc by checking for sys/sysctl.h with
has_include, which in turn is only used if it is supported. This should mean
that this commit does not prevent any configuration that worked before from working.

libavutil/cpu.c

@@ -39,8 +39,16 @@
 #include <sys/param.h>
 #endif
 #include <sys/types.h>
+
+
+#if __cplusplus >= 201703L
+#if __has_include(<sys/sysctl.h>)
 #include <sys/sysctl.h>
 #endif
+#else
+#include <sys/sysctl.h>
+#endif
+#endif
 #if HAVE_UNISTD_H
 #include <unistd.h>
 #endif