define inline for Visual Studio

In Visual Studio, inline is available in C++ only, however __inline is available for C, see
http://msdn.microsoft.com/en-us/library/z8y1yy88.aspx



Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Dr Stephen Henson <steve@openssl.org>
(cherry picked from commit f511b25a)