Use 'unsigned long' for _BitScanReverse parameter

New clang warnings in chromium reveal an incompatible pointer being
passed.

Change-Id: I0d893993726ecf899f467c16175bfb8c76329731
https://code.google.com/p/chromium/issues/detail?id=504696

vp8/encoder/x86/quantize_ssse3.c

diff --git a/vp8/encoder/x86/quantize_ssse3.c b/vp8/encoder/x86/quantize_ssse3.c
index 448217f..14282db 100644
--- a/vp8/encoder/x86/quantize_ssse3.c
+++ b/vp8/encoder/x86/quantize_ssse3.c
@@ -17,7 +17,7 @@
 #include <intrin.h>
 #pragma intrinsic(_BitScanReverse)
 static int bsr(int mask) {
-  int eob;
+  unsigned long eob;
   _BitScanReverse(&eob, mask);
   eob++;
   if (mask == 0)