fix compile warnings

src/sdl2_gfx/framerate.rs

@@ -10,6 +10,7 @@ mod ll {
     /* automatically generated by rust-bindgen */
 
     use libc::*;
+    #[repr(C)]
     pub struct FPSmanager {
         pub framecount: uint32_t,
         pub rateticks: c_float,

src/sdl2_gfx/primitives.rs

@@ -457,7 +457,7 @@ impl DrawRenderer for Renderer {
         if ret == 0 { Ok(()) }
         else { Err(get_error()) }
     }
-    #[allow(unused_variable)]
+    #[allow(unused_variables)]
     fn textured_polygon<C: ToColor>(&self, vx: &[i16], vy: &[i16], texture: &Surface, texture_dx: i16, texture_dy: i16, color: C) -> SdlResult<()> {
         unimplemented!()
     }