sys/windows: add winmm bindings file

core/sys/windows/winmm.odin

@@ -0,0 +1,9 @@
+// +build windows
+package sys_windows
+
+foreign import winmm "system:Winmm.lib"
+
+@(default_calling_convention="stdcall")
+foreign winmm {
+	timeBeginPeriod :: proc(uPeriod: UINT) -> MMRESULT ---
+}