Include system headers consistently

Problem: A few files include system headers after local project
headers.

Solution: For consistency to rest of flux-core, always include system
headers first.

Author: chu11

src/common/libflux/test/version.c

@@ -8,12 +8,12 @@
  * SPDX-License-Identifier: LGPL-3.0
 \************************************************************/
 
+#include <string.h>
+
 #include "src/common/libflux/version.h"
 #include "src/common/libtap/tap.h"
 #include "ccan/str/str.h"
 
-#include <string.h>
-
 int main (int argc, char *argv[])
 {
     const char *s;

src/common/libpmi/test/keyval.c

@@ -8,13 +8,13 @@
  * SPDX-License-Identifier: LGPL-3.0
 \************************************************************/
 
+#include <string.h>
+#include <ctype.h>
+
 #include "src/common/libtap/tap.h"
 #include "src/common/libpmi/keyval.h"
 #include "ccan/str/str.h"
 
-#include <string.h>
-#include <ctype.h>
-
 static char *valid[] = {
     "key1=val1",
     "key1=val1 ",

src/common/libutil/test/stdlog.c

@@ -8,14 +8,14 @@
  * SPDX-License-Identifier: LGPL-3.0
 \************************************************************/
 
+#include <string.h>
+#include <ctype.h>
+
 #include "src/common/libtap/tap.h"
 #include "src/common/libutil/wallclock.h"
 #include "src/common/libutil/stdlog.h"
 #include "ccan/str/str.h"
 
-#include <string.h>
-#include <ctype.h>
-
 static char *valid[] = {
     "<1>1 - - - - - - message",
     "<23>1 - - - - - - message",

src/common/libutil/test/wallclock.c

@@ -8,13 +8,13 @@
  * SPDX-License-Identifier: LGPL-3.0
 \************************************************************/
 
+#include <string.h>
+#include <ctype.h>
+
 #include "src/common/libtap/tap.h"
 #include "src/common/libutil/wallclock.h"
 #include "src/common/libutil/stdlog.h"
 
-#include <string.h>
-#include <ctype.h>
-
 int main(int argc, char** argv)
 {
     char buf[WALLCLOCK_MAXLEN];