editors/plume-creator-devel: unbreak build with Clang 6 (C++14 by default)

In file included from externals/hunspell/affixmgr.cxx:12:
externals/hunspell/affentry.hxx:30:94: error: cannot initialize a parameter of type 'unsigned short' with an rvalue of type 'nullptr_t'

struct hentry *      check_twosfx(const char * word, int len, char in_compound, const FLAG needflag = NULL);
                                                                                           ^          ~~~~

externals/hunspell/affentry.hxx:30:94: note: passing argument to parameter 'needflag' here
externals/hunspell/affentry.hxx:93:103: error: cannot initialize a parameter of type 'unsigned short' with an rvalue of type 'nullptr_t'

struct hentry *   check_twosfx(const char * word, int len, int optflags, PfxEntry* ppfx, const FLAG needflag = NULL);
                                                                                                    ^          ~~~~

Reported by: pkg-fallout
Obtained from: upstream (Hunspell 1.4.0)