Move smath.hpp to smath/smath.hpp

This is done in preparation for optional interop headers for different
libraries.

Signed-off-by: Slendi <slendi@socopon.com>

src/modules/smath.cppm

@@ -1,6 +1,6 @@
 module;
 
-#include "smath.hpp"
+#include "smath/smath.hpp"
 
 export module smath;