Make all required includes explicit

Information as seen by IWYU 0.12 + clang 9 on Linux

linux/LinuxProcessList.h

@@ -7,7 +7,14 @@ Released under the GNU GPLv2, see the COPYING file
 in the source distribution for its full text.
 */
 
+#include "config.h"
+
+#include <stdbool.h>
+#include <sys/types.h>
+
+#include "Hashtable.h"
 #include "ProcessList.h"
+#include "UsersTable.h"
 #include "zfs/ZfsArcStats.h"
 
 typedef struct CPUData_ {