dsp/lossless: Average2, make a constant unsigned

use 'u' rather than the unnecessary 'l' as a suffix. this prevents a
conversion warning with some toolchains

Change-Id: I21c33ce08819b3c839c75e03a8f7f3a6041d0695
diff --git a/src/dsp/lossless.c b/src/dsp/lossless.c
index 5ca7759..8fae7f0 100644
--- a/src/dsp/lossless.c
+++ b/src/dsp/lossless.c
@@ -35,7 +35,7 @@
 }
 
 static WEBP_INLINE uint32_t Average2(uint32_t a0, uint32_t a1) {
-  return (((a0 ^ a1) & 0xfefefefeL) >> 1) + (a0 & a1);
+  return (((a0 ^ a1) & 0xfefefefeu) >> 1) + (a0 & a1);
 }
 
 static WEBP_INLINE uint32_t Average3(uint32_t a0, uint32_t a1, uint32_t a2) {