test(api-watchdog): Fix errors in compiling watchdog greentea tests

The latest rebase of the watchdog feature branch introduced errors in compiling
watchdog tests due to missing headers.

- Watchdog HAL API test
  Include missing header files to main.cpp (mbed_wait_api.h, stdlib.h).

- Watchdog HAL API timing test
  Include missing header files to main.cpp (us_ticker_api.h).

- Watchdog Driver API test
  Include missing header files to main.cpp (mbed_wait_api.h, stdlib.h).
  Inject mbed namespace into main.cpp to fix Watchdog name resolution.

TESTS/mbed_drivers/watchdog/main.cpp

@@ -18,13 +18,16 @@
 #endif
 
 #define __STDC_LIMIT_MACROS
-#include <stdint.h>
-#include "greentea-client/test_env.h"
-#include "utest/utest.h"
-#include "unity/unity.h"
 #include "drivers/Watchdog.h"
+#include "greentea-client/test_env.h"
+#include "mbed_wait_api.h"
+#include "unity/unity.h"
+#include "utest/utest.h"
 #include "Watchdog_tests.h"
 
+#include <stdint.h>
+#include <stdlib.h>
+
 /* This is platform specific and depends on the watchdog timer implementation,
  * e.g. STM32F4 uses 32kHz internal RC oscillator to clock the IWDG, so
  * when the prescaler divider is set to max value of 256 the resolution
@@ -50,6 +53,8 @@ using utest::v1::Case;
 using utest::v1::Specification;
 using utest::v1::Harness;
 
+using namespace mbed;
+
 void test_max_timeout_is_valid()
 {
     Watchdog watchdog;

TESTS/mbed_hal/watchdog/main.cpp

@@ -18,11 +18,14 @@
 #endif
 
 #include "greentea-client/test_env.h"
-#include "utest/utest.h"
-#include "unity/unity.h"
 #include "hal/watchdog_api.h"
+#include "mbed_wait_api.h"
+#include "unity/unity.h"
+#include "utest/utest.h"
 #include "watchdog_api_tests.h"
 
+#include <stdlib.h>
+
 /* This is platform specific and depends on the watchdog timer implementation,
  * e.g. STM32F4 uses 32kHz internal RC oscillator to clock the IWDG, so
  * when the prescaler divider is set to max value of 256 the resolution

TESTS/mbed_hal/watchdog_timing/main.cpp

@@ -18,9 +18,10 @@
 #endif
 
 #include "greentea-client/test_env.h"
-#include "utest/utest.h"
-#include "unity/unity.h"
 #include "hal/watchdog_api.h"
+#include "unity/unity.h"
+#include "us_ticker_api.h"
+#include "utest/utest.h"
 #include "watchdog_timing_tests.h"
 
 #define MSG_VALUE_DUMMY "0"