dialog: fix macro redefinition

dialog.h defines MIN and MAX (making sure to undefine the previous
macros if it already exists), but sys/param.h also defines those
macros (without guards) and is included after dialog.h resulting
in both gcc and clang complaining about macro redefiniton

While clang do accept -Wno-macro-redefined to ignore the redefinition
warning, gcc does not [1]

Undefine both macros prior inclusion of sys/param.h to avoid the warning

Reported by: arichardson