wmgtemp: Use 'char' instead of 'unsigned char' for bit mask.

wmgtemp: Use 'char' instead of 'unsigned char' for bit mask.

Avoids compiler warning:

    src/wmgtemp.c: In function ‘main’:
    src/wmgtemp.c:202:50: warning: pointer targets in passing argument 4 of ‘openXwindow’ differ in signedness [-Wpointer-sign]
       openXwindow(argc, argv, wmgtemp_interface_xpm, wmgtemp_interface_mask_bits,
                                                  ^
    In file included from src/wmgtemp.c:15:0:
    /usr/include/libdockapp/wmgeneral.h:78:6: note: expected ‘char *’ but argument is of type ‘unsigned char *’
     void openXwindow(int argc, char *argv[], char **, char *, int, int);
          ^