Fix MinGW requiring fpermissive to compile

[ncruces]

This is the error I get otherwise:

../../src/nfd_win.cpp: In function 'nfdresult_t NFD_OpenDialog(const nfdchar_t*, const nfdchar_t*, nfdchar_t**)':
../../src/nfd_win.cpp:448:1: error: jump to label 'end' [-fpermissive]
 end:
 ^~~
../../src/nfd_win.cpp:379:14: note:   from here
         goto end;
              ^~~
../../src/nfd_win.cpp:383:13: note:   crosses initialization of 'HRESULT result'
     HRESULT result = ::CoCreateInstance(::CLSID_FileOpenDialog, NULL,
             ^~~~~~

[ncruces]

Closing, hadn't seen Submitting Pull Requests.

Branches are even, but I'll stick to the rules: #63