Move volume helper functions

I moved these to a volume.h file to keep them separate from controls.

gui/buttons/volume.cpp

@@ -1,5 +1,5 @@
 
-#include <libsaria/controls.h>
+#include <libsaria/volume.h>
 #include <libsaria/print.h>
 #include <ocarina/button.h>
 

include/libsaria/controls.h

@@ -7,6 +7,3 @@ void libsaria_pause();
 void libsaria_stop();
 
 void libsaria_load(string);
-
-void libsaria_set_volume(double);
-double libsaria_get_volume();

include/libsaria/volume.h

@@ -0,0 +1,3 @@
+
+void libsaria_set_volume(double);
+double libsaria_get_volume();