Fix warning in the STL code

The fast_float is an external library, so move it to the
system includes which has less strict compiler flags applied.

This matches how other IO module use this library as well.

Pull Request: blender/blender#106892

source/blender/io/stl/CMakeLists.txt

@@ -14,12 +14,11 @@ set(INC
   ../../makesrna
   ../../nodes
   ../../windowmanager
-  ../../../../extern/fast_float
   ../../../../intern/guardedalloc
 )
 
 set(INC_SYS
-
+  ../../../../extern/fast_float
 )
 
 set(SRC