fix "expected *mut u8, found *mut i8" error

Author: sbernauer

vncserver/src/rfb/mod.rs

@@ -40,7 +40,8 @@ pub fn rfb_screen_cleanup(ptr: rfbScreenInfoPtr) {
 
 pub fn rfb_framebuffer_malloc(ptr: rfbScreenInfoPtr, fb_size: u64) {
     unsafe {
-        (*ptr).frameBuffer = malloc(fb_size as ::std::os::raw::c_ulong) as *mut i8;
+        (*ptr).frameBuffer =
+            malloc(fb_size as ::std::os::raw::c_ulong) as *mut ::std::os::raw::c_char;
     }
 }