src/mainboard: Remove unused 'include <stdlib.h>'

Found using following commande:
diff <(git grep -l '#include <stdlib.h>' -- src/) <(git grep -l '
memalign\|malloc\|free' -- src/) |grep -v vendorcode |grep '<'

Change-Id: Ib2ee840a10de5c10d57aa7a75b805ef69dc8da84
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41241
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/biostar/a68n_5200/BiosCallOuts.c

@@ -4,7 +4,6 @@
 #include <northbridge/amd/agesa/BiosCallOuts.h>
 #include <northbridge/amd/agesa/state_machine.h>
 #include <FchPlatform.h>
-#include <stdlib.h>
 
 #include "imc.h"
 

src/mainboard/biostar/a68n_5200/buildOpts.c

@@ -12,7 +12,6 @@
  *
  */
 
-#include <stdlib.h>
 #include <AGESA.h>
 
 #define INSTALL_FT3_SOCKET_SUPPORT           TRUE