Contribs: update SDL to 1.2.15

(cherry picked from commit 1ff2b3fb)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/sdl/SHA512SUMS

-61ab65661492038d319b83521a27dc80dc71c76b63965ec094c9f53fbb76466e612d119297f9797d62147a0641808136181d0926e8082cb9b3e8d2a8e688a8b2  SDL-1.2.14.tar.gz
+ac392d916e6953b0925a7cbb0f232affea33339ef69b47a0a7898492afb9784b93138986df53d6da6d3e2ad79af1e9482df565ecca30f89428be0ae6851b1adc  SDL-1.2.15.tar.gz

contrib/src/sdl/rules.mak

 # SDL
 
-SDL_VERSION := 1.2.14
+SDL_VERSION := 1.2.15
 SDL_URL := http://www.libsdl.org/release/SDL-$(SDL_VERSION).tar.gz
 
 #PKGS += sdl