oldXMenu: add missing #include <string.h>

Some of the files in oldXMenu use functions from string.h without
including that header which results in compile warnings:

	ChgPane.c:46:5: warning: implicit declaration of function ‘strlen’
	ChgPane.c:46:20: warning: incompatible implicit declaration of
		built-in function ‘strlen’
	ChgSel.c:62:2: warning: implicit declaration of function ‘strlen’
	ChgSel.c:62:17: warning: incompatible implicit declaration of built-in
		function ‘strlen’
	Create.c:220:5: warning: implicit declaration of function ‘strcmp’
	InsPane.c:65:5: warning: implicit declaration of function ‘strlen’
	InsPane.c:65:20: warning: incompatible implicit declaration of
		built-in function ‘strlen’
	InsSel.c:68:5: warning: implicit declaration of function ‘strlen’
	InsSel.c:68:20: warning: incompatible implicit declaration of built-in
		function ‘strlen’
	InsSel.c:75:5: warning: implicit declaration of function ‘strcmp’

Add the necessary ‘#include <string.h>’.

oldXMenu/ChgPane.c, oldXMenu/ChgSel.c, oldXMenu/Create.c, oldXMenu/InsPane.c,
oldXMenu/InsSel.c: add missing #include <string.h>

oldXMenu/ChgPane.c

@@ -14,6 +14,7 @@
  */
 
 #include "XMenuInt.h"
+#include <string.h>
 
 int
 XMenuChangePane(register XMenu *menu, register int p_num, char *label)

oldXMenu/ChgSel.c

@@ -14,6 +14,7 @@
  */
 
 #include "XMenuInt.h"
+#include <string.h>
 
 int
 XMenuChangeSelection(Display *display, register XMenu *menu, register int p_num, register int s_num, char *data, int data_sw, char *label, int label_sw)

oldXMenu/Create.c

@@ -31,6 +31,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include "XMenuInt.h"
 #include <stdlib.h>
+#include <string.h>
 
 #ifdef EMACS_BITMAP_FILES
 #include "../src/bitmaps/dimple1.xbm"

oldXMenu/InsPane.c

@@ -15,6 +15,7 @@
  */
 
 #include "XMenuInt.h"
+#include <string.h>
 
 int
 XMenuInsertPane(register XMenu *menu, register int p_num, char *label, int active)

oldXMenu/InsSel.c

@@ -14,6 +14,7 @@
  */
 
 #include "XMenuInt.h"
+#include <string.h>
 
 int
 XMenuInsertSelection(register XMenu *menu, register int p_num, register int s_num, char *data, char *label, int active)