os.h: change elif _WIN32 to elif defined(_WIN32)

This symbol is only defined (with the value 1) when building
for the Windows target, so we need to ifdef, not if.
Signed-off-by: Ralph Giles <giles@thaumas.net>

lib/os.h

@@ -30,7 +30,7 @@
 
 #  ifdef __GNUC__
 #    define STIN static __inline__
-#  elif _WIN32
+#  elif defined(_WIN32)
 #    define STIN static __inline
 #  else
 #    define STIN static