fix warnings about implicit declaration of strtol()

This commit fixes several warnings like this one:

In file included from ../../../src/libsmpp34/test_apps/smpp.c:40:
test_apps/smpp.c: In function ‘do_smpp_send_message2’:
test_apps/esme.h:32:16: warning: implicit declaration of function ‘strtol’;
                                 did you mean ‘strtok’? [-Wimplicit-function-declaration]
   32 |          dst = strtol((char*)clave, NULL, 10);\
      |                ^~~~~~
test_apps/smpp.c:223:5: note: in expansion of macro ‘GET_PROP_INT’
  223 |     GET_PROP_INT( req.source_addr_ton, p, "source_addr_ton" );
      |     ^~~~~~~~~~~~

Change-Id: I16d9e0171d4c26f721c300e22c78b77f6db4895c
diff --git a/test_apps/esme.c b/test_apps/esme.c
index 77fb198..5d9dafd 100644
--- a/test_apps/esme.c
+++ b/test_apps/esme.c
@@ -23,6 +23,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
 #include <libxml/xmlmemory.h>
diff --git a/test_apps/sendwp.c b/test_apps/sendwp.c
index b719c45..f75af76 100644
--- a/test_apps/sendwp.c
+++ b/test_apps/sendwp.c
@@ -23,6 +23,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
 #include <libxml/xmlmemory.h>
diff --git a/test_apps/smpp.c b/test_apps/smpp.c
index c764907..2cdb138 100644
--- a/test_apps/smpp.c
+++ b/test_apps/smpp.c
@@ -23,6 +23,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
 #include <sys/types.h>
diff --git a/test_apps/tcp.c b/test_apps/tcp.c
index 4cf681d..3051df2 100644
--- a/test_apps/tcp.c
+++ b/test_apps/tcp.c
@@ -28,6 +28,7 @@
 #include <arpa/inet.h>
 #include <netdb.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
 #include <libxml/xmlmemory.h>