[Tizen 6.0] Support build with GCC 9.2

[Tizen 6.0] Support build with GCC 9.2

wp-loader.cpp:34:49: error: 'function' in namespace 'std' does not name
                     a template type
   34 | int getValueCstr(std::string &value, const std::function<int(const char **)> &getfunc)
      |                                                 ^~~~~~~~
wp-loader.cpp:29:1: note: 'std::function' is defined in header '<functional>';
                    did you forget to '#include <functional>'?
   28 | #include "common/exception.h"
  +++ |+#include <functional>

Change-Id: I7d4d627cfef2356a2061f6b5e7051e34a68fcee1
Signed-off-by: Mikhail Kashkarov <m.kashkarov@partner.samsung.com>
Signed-off-by: Sangwan Kwon <sangwan.kwon@samsung.com>