rm std::ptr_fun for gcc12 compliance

[APN-Pucky]

This fixes following error (cf. https://stackoverflow.com/a/217605):

src/utils/utils.h: In function ‘std::string& ltrim(std::string&)’:
src/utils/utils.h:205:87: error: ‘std::pointer_to_unary_function<_Arg, _Result> std::ptr_fun(_Result (*)(_Arg)) [with _Arg = int; _Result = int]’ is deprecated: use 'std::function' instead [-Werror=deprecated-declarations]
  205 | s.erase(s.begin(), std::find_if(s.begin(), s.end(), std::not1(std::ptr_fun<int, int>(std::isspace))));
      |                                                               ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~

[...]

/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/bits/stl_function.h:1126:5: note: declared here
 1126 |     ptr_fun(_Result (*__x)(_Arg))
      |     ^~~~~~~
src/utils/utils.h: In function ‘std::string& rtrim(std::string&)’:
src/utils/utils.h:211:78: error: ‘std::pointer_to_unary_function<_Arg, _Result> std::ptr_fun(_Result (*)(_Arg)) [with _Arg = int; _Result = int]’ is deprecated: use 'std::function' instead [-Werror=deprecated-declarations]
  211 |   s.erase(std::find_if(s.rbegin(), s.rend(), std::not1(std::ptr_fun<int, int>(std::isspace))).base(), s.end());