Clean up unnecessary exports and add static keywords

This patch cleans up unnecessary exports from header files (there were
only a few), as well as adds the static keyword to all symbols that
aren't exported and are specific to a file. This helps the linker out in
not doing any unnecessary work, speeding it up and avoiding silent
symbol conflicts (otherwise two symbols with the same name (and
type/signature in C++) would quietly resolve as okay by the linker).

desktop_version/src/Music.cpp

@@ -8,7 +8,7 @@
 #include "Map.h"
 #include "UtilityClass.h"
 
-void songend();
+static void songend();
 
 musicclass::musicclass()
 {
@@ -158,7 +158,7 @@ void musicclass::init()
 	}
 }
 
-void songend()
+static void songend()
 {
 	extern musicclass music;
 	music.songEnd = SDL_GetPerformanceCounter();