Fix #1486: build fails on Windows with VS2013 toolset (v120)

Fix #1486: build fails on Windows with VS2013 toolset (v120)

Build fails on compiling xxhash.c having a fuction with "inline" specifier.
"inline" is a c99 keyword and c99 is not yet (fully) supported with MSVC toolset v120:
"The inline keyword is available only in C++. The __inline and __forceinline
keywords are available in both C and C++. For compatibility with previous versions,
_inline is a synonym for __inline."
(Source: http://msdn.microsoft.com/en-us/library/z8y1yy88.aspx)

This fix adds a patch that replaces "inline" with "__inline" in xxhash.c