Fix wxReadlink signature to return ssize_t

Introduced in 53bd139, wxReadlink() trivially wraps readlink(), but
returned int instead of ssize_t as defined for readlink() by POSIX.

Fixes "Implicit conversion loses integer precision: 'ssize_t' (aka
'long') to 'int'" on platforms with sizeof(ssize_t)>sizeof(int).

src/common/filename.cpp

@@ -1698,7 +1698,7 @@ wxFileName wxFileName::ResolveLink()
         bufSize = st.st_size + 1;
 
     char buf[bufSize];
-    int result = wxReadlink(link, buf, bufSize - 1);
+    ssize_t result = wxReadlink(link, buf, bufSize - 1);
 
     if ( result != -1 )
     {