cellular: AT: fix build on ARMCC5

ARMC5 failed to compile the code with debug-profile as va_list is
not getting properly defined by <cstdarg>. Convert to stdarg.h which
works as expected.

While here, remove one extra copy of #include "PlatformMutex.h".

Error being fixed:
--8<--8<--8<--
Compile [ 81.8%]: ATHandler.cpp
[Error] ATHandler.h@552,0:  #20: identifier "va_list" is undefined
[Error] ATHandler.cpp@1226,0:  #147: declaration is incompatible with "void mbed::ATHandler::handle_args(const char *, <error-type>)"  (declared at line 552 of "./mbed-os/features/cellular/framework/AT/ATHandler.h")
[ERROR] "./mbed-os/features/cellular/framework/AT/ATHandler.h", line 552: Error:  #20: identifier "va_list" is undefined
"./mbed-os/features/cellular/framework/AT/ATHandler.cpp", line 1226: Error:  #147: declaration is incompatible with "void mbed::ATHandler::handle_args(const char *, <error-type>)"  (declared at line 552 of "./mbed-os/features/cellular/framework/AT/ATHandler.h")
./mbed-os/features/cellular/framework/AT/ATHandler.cpp: 0 warnings, 2 errors

Author: TeroJaasko

features/cellular/framework/AT/ATHandler.h

@@ -24,9 +24,9 @@
 #include "PlatformMutex.h"
 #include "nsapi_types.h"
 
-#include "PlatformMutex.h"
 #include "Callback.h"
-#include <cstdarg>
+
+#include <stdarg.h>
 
 namespace mbed {