MSVC: remove gcc-ism for Visual Studio

MSVC does not have __attribute__((foo))

src/plat.h

@@ -68,8 +68,13 @@
 #endif
 
 
+#ifndef _MSC_VER
 /* A hack (GCC-specific?) to allow us to ignore unused parameters. */
 #define UNUSED(arg)	__attribute__((unused))arg
+#else
+/* MSVC does not have __attribute__((unused)). */
+#define UNUSED(arg) arg
+#endif
 
 /* Return the size (in wchar's) of a wchar_t array. */
 #define sizeof_w(x)	(sizeof((x)) / sizeof(wchar_t))