Import ctype.h in check_strftime.sh

[Schievel1]

Clang16 will not allow -Wimplicit-function-declaration will results in the error

checking if strftime is GNU or Non-GNU... /tmp/strftime_try.c:16:8: error: call to undeclared library function 'isdigit' with type
      'int (int)'; ISO C99 and later do not support implicit function declarations
      [-Werror,-Wimplicit-function-declaration]
  if (!isdigit(buf[1]))
       ^

during configure. While this does not stop the configure, it could result in unexpected behavior.
It's enough to import the right library into this check.

Signed-off-by: Pascal Jäger [email protected]