MSVC: lower C4100 warning level from 4 to 3

This better aligns with OSX/Linux warnings.

Although `__pragma(warning(suppress:4100))` is not the same as
`__attribute__((__unused__))` in gcc (which only affects the attribute
instead of the line), it still seems to be better to use it than to
hide the warning entirely.

source/blender/python/intern/stubs.c

@@ -12,6 +12,9 @@
 #if defined(__GNUC__) || defined(__clang__)
 #  pragma GCC diagnostic error "-Wmissing-prototypes"
 #  pragma GCC diagnostic ignored "-Wunused-parameter"
+#elif defined(_MSC_VER)
+/* Suppress unreferenced formal parameter warning. */
+#  pragma warning(disable : 4100)
 #endif
 
 /* python, will come back */