commonlib: Replace 'stdlib' with 'stddef'

Include <stddef.h> since we need it for 'size_t'.
Unused <stdlib.h> found using:
diff <(git grep -l '#include <stdlib.h>' -- src/) <(git grep -l 'memalign(\|malloc(\|calloc(\|free(' -- src/)

Change-Id: I3c2668013c16d6771268e8739b1370968c2e120b
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/60620
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <martinroth@google.com>

src/commonlib/fsp_relocate.c

@@ -18,7 +18,7 @@
 #pragma pack(pop)
 
 #include <commonlib/helpers.h>
-#include <stdlib.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <string.h>