Explicitly cast -1 to fpos_t in our wxFtell() definition

Apparently in at least some gcc versions fpos_t is not the same thing as
long long int, resulting in compilation problems due to using different
types for the ternary operator arguments.

include/wx/filefn.h

@@ -162,7 +162,7 @@ enum wxPosixPermissions
             inline long long wxFtell(FILE* fp)
             {
                 fpos_t pos;
-                return fgetpos(fp, &pos) == 0 ? pos : -1LL;
+                return fgetpos(fp, &pos) == 0 ? pos : static_cast<fpos_t>(-1LL);
             }
         #else
             #define wxFtell ftello64