Fix building in ISO C23

Building with GCC 15, which defaults to ISO C23, failed like this:

    In file included from lib/SDL_perl.xs:32:
    lib/SDL_perl.c:654:13: error: conflicting types for ‘boot_SDL’; have ‘void(PerlInterpreter *, CV *)’ {aka ‘void(struct interpreter *, struct cv *)’}
      654 | XS_EXTERNAL(boot_SDL); /* prototype to pass -Wmissing-prototypes */
	  |             ^~~~~~~~
    /usr/lib64/perl5/CORE/XSUB.h:149:34: note: in definition of macro ‘XS_EXTERNAL’
      149 | #  define XS_EXTERNAL(name) void name(pTHX_ CV* cv __attribute__unused__)
	  |                                  ^~~~
    lib/SDL_perl.xs:147:6: note: previous declaration of ‘boot_SDL’ with type ‘void(void)’
      147 | void boot_SDL();
	  |      ^~~~~~~~
    lib/SDL_perl.c:655:13: error: conflicting types for ‘boot_SDL’; have ‘void(PerlInterpreter *, CV *)’ {aka ‘void(struct interpreter *, struct cv *)’}
      655 | XS_EXTERNAL(boot_SDL)
	  |             ^~~~~~~~
    /usr/lib64/perl5/CORE/XSUB.h:149:34: note: in definition of macro ‘XS_EXTERNAL’
      149 | #  define XS_EXTERNAL(name) void name(pTHX_ CV* cv __attribute__unused__)
	  |                                  ^~~~
    lib/SDL_perl.xs:147:6: note: previous declaration of ‘boot_SDL’ with type ‘void(void)’
      147 | void boot_SDL();
	  |      ^~~~~~~~

The cause is a mismatch between how boot_SDL() was declared and used
in src/SDL.xs and how Perl generates a boot function for XS packages.
This patch fixes it by passing current Perl interpreter and, probably
ignored, cv argument.

Resolves: #294
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2341036

src/SDL.xs

@@ -144,7 +144,7 @@ sdl_perl_atexit (void)
 	SDL_Quit();
 }
 
-void boot_SDL();
+XS(boot_SDL);
 void boot_SDL__OpenGL();
 
 XS(boot_SDL_perl)
@@ -155,7 +155,7 @@ XS(boot_SDL_perl)
 #endif
 	PL_perl_destruct_level = 2;
 	GET_TLS_CONTEXT
-	boot_SDL();
+	boot_SDL(aTHX_ cv);
 
 #if defined WINDOWS || defined WIN32
   SDL_RegisterApp ("SDLPerl App", 0, GetModuleHandle (NULL));