Commit 5dddf7c8 (Add block-sha1 in favour of the mozilla routines
2010-04-14) introduced the "bswap.h" header file which contains
an inline function (default_swab32()).  The msvc compiler does
not support the inline keyword which causes the build to fail
with a syntax error.

However, msvc does support inline functions using the __inline
keyword language extension.  We already have the GIT_INLINE()
macro that allows us to hide this syntatic difference. In order
to fix the build, we simply use GIT_INLINE() in the definition
of the default_swab32() function.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>