Add SDL_main.h includes to tests

.. including cmake/test/*.c, whatever that does

(this is needed now because SDL.h doesn't include SDL_main.h anymore)

cmake/test/main_cli.c

@@ -1,5 +1,6 @@
 #define SDL_MAIN_HANDLED
 #include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
 #include <stdio.h>
 
 int main(int argc, char *argv[]) {

cmake/test/main_gui.c

@@ -1,4 +1,5 @@
 #include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
 #include <stdio.h>
 
 int main(int argc, char *argv[]) {

cmake/test/main_lib.c

@@ -1,4 +1,6 @@
 #include <SDL3/SDL.h>
+#define SDL_MAIN_HANDLED /* don't drag in header-only SDL_main implementation */
+#include <SDL3/SDL_main.h>
 #include <stdio.h>
 
 #include EXPORT_HEADER