meson: Add missing SDL dependency to system/main.c

meson: Add missing SDL dependency to system/main.c

When building QEMU configure with --disable-gtk --disable-cocoa
on macOS we get:

  User interface
    Cocoa support                   : NO
    SDL support                     : YES 2.30.5
    SDL image support               : NO
    GTK support                     : NO
    pixman                          : YES 0.42.2
    VTE support                     : NO
    PNG support                     : YES 1.6.43
    VNC support                     : YES
    VNC SASL support                : YES
    VNC JPEG support                : YES 3.0.3
    spice protocol support          : YES 0.14.4
      spice server support          : NO
    curses support                  : YES
    brlapi support                  : NO

  User defined options
    cocoa                           : disabled
    docs                            : disabled
    gtk                             : disabled

  ../system/main.c:30:10: fatal error: 'SDL.h' file not found
     30 | #include <SDL.h>
        |          ^~~~~~~
  1 error generated.

Fix by adding the SDL dependency to main.c it's CFLAGS contains
the SDL include directory.

Fixes: 64ed6f92ff ("meson: link emulators without Makefile.target")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20241120114943.85080-1-philmd@linaro.org>