change int8 to signed char for compatibility with chromium type

BUG=none
TEST=none
Review URL: http://webrtc-codereview.appspot.com/318012

git-svn-id: http://libyuv.googlecode.com/svn/trunk@105 16f28f9a-4ce2-e073-06de-1de4eb20be90

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 103
+Version: 104
 License: BSD
 License File: LICENSE
 

include/libyuv/basic_types.h

@@ -57,7 +57,7 @@ typedef int int32;
 typedef unsigned short uint16;
 typedef short int16;
 typedef unsigned char uint8;
-typedef char int8;
+typedef signed char int8;
 #endif  // INT_TYPES_DEFINED
 
 // Detect compiler is for x86 or x64.