Fix warning for Android NDK compiler: "function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]"

https://stackoverflow.com/questions/42125/warning-error-function-declaration-isnt-a-prototype
In C int foo() and int foo(void) are different functions. int foo() accepts an arbitrary number of arguments, while int foo(void) accepts 0 arguments. In C++ they mean the same thing.

src/render/SDL_d3dmath.c

@@ -26,7 +26,7 @@
 
 /* Direct3D matrix math functions */
 
-Float4X4 MatrixIdentity()
+Float4X4 MatrixIdentity(void)
 {
     Float4X4 m;
     SDL_zero(m);