Warnings: fix a documentation warning and missing prototypes

src/render/software/SDL_render_sw.c

@@ -820,7 +820,7 @@ SW_CreateRendererForSurface(SDL_Surface * surface)
     return renderer;
 }
 
-SDL_Renderer *
+static SDL_Renderer *
 SW_CreateRenderer(SDL_Window * window, Uint32 flags)
 {
     SDL_Surface *surface;

src/thread/pthread/SDL_systhread.c

@@ -49,6 +49,7 @@
 #include "SDL_log.h"
 #include "SDL_platform.h"
 #include "SDL_thread.h"
+#include "SDL_system.h"
 #include "../SDL_thread_c.h"
 #include "../SDL_systhread.h"
 #ifdef __ANDROID__