libmpeg2: enable SSE2

SSE2 support was added 13 years ago ([1]).

an SSE3 flag also exists, but there does not appear to be any actual SSE3
implementation and development seems to have ground to a halt years ago, so
i've not bothered adding use of that flag.

[1]: videolan/libmpeg2@c80d1dc2

modules/codec/libmpeg2.c

@@ -203,6 +203,8 @@ static int OpenDecoder( vlc_object_t *p_this )
         i_accel |= MPEG2_ACCEL_X86_MMX;
     if( vlc_CPU_MMXEXT() )
         i_accel |= MPEG2_ACCEL_X86_MMXEXT;
+    if( vlc_CPU_SSE2() )
+        i_accel |= MPEG2_ACCEL_X86_SSE2;
 #elif defined( __powerpc__ ) || defined( __ppc__ ) || defined( __ppc64__ )
     if( vlc_CPU_ALTIVEC() )
         i_accel |= MPEG2_ACCEL_PPC_ALTIVEC;