msvc: fix pointer type warning in BitsLog2Floor

_BitScanReverse() takes an unsigned long*
http://msdn.microsoft.com/en-us/library/fbxyd7zd.aspx

fixes:
C4057: 'function': 'unsigned long *' differs in indirection to slightly
different base types from 'uint32_t *'

fixes issue #253

(cherry picked from commit 0250dfcc19)

Change-Id: I0101ef7be18c7ed188b35e9b17e7f71290953786

src/utils/utils.h

@@ -90,7 +90,7 @@ static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
 #pragma intrinsic(_BitScanReverse)
 
 static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
-  uint32_t first_set_bit;
+  unsigned long first_set_bit;
   _BitScanReverse(&first_set_bit, n);
   return first_set_bit;
 }