Amends the following compiler warning

 Display.c: In function ‘nxagentMakeIcon’:
 Display.c:1949:18: warning: assignment from incompatible pointer type
      agentIconData=x2goagentIconData;
                  ^
 Display.c:1954:18: warning: assignment from incompatible pointer type
      agentIconData=nxagentIconData;
                  ^
 Display.c:2001:41: warning: passing argument 3 of ‘XpmCreatePixmapFromData’ from incompatible pointer type
                                          agentIconData,
                                          ^
 In file included from Icons.h:22:0,
                  from Display.c:67:
 X11/include/xpm_nxagent.h:242:6: note: expected ‘char **’ but argument is of type ‘char *’
  FUNC(XpmCreatePixmapFromData, int, (Display *display,
       ^
 X11/include/xpm_nxagent.h:230:32: note: in definition of macro ‘FUNC’
  #define FUNC(f, t, p) extern t f p