Include malloc.h in libc/stdlib/aligned_alloc.c

Include malloc.h in libc/stdlib/aligned_alloc.c

Without this, for a bare-iron/simulator target such as cris-elf,
you'll see, at newlib build time:

/x/gccobj/./gcc/xgcc -B/x/gccobj/./gcc/ <many options elided> -c -o lib_a-aligned_alloc.o \
 `test -f 'aligned_alloc.c' || echo '/y/newlib/libc/stdlib/'`aligned_alloc.c
/y/newlib/libc/stdlib/aligned_alloc.c: In function 'aligned_alloc':
/y/newlib/libc/stdlib/aligned_alloc.c:35:10: warning: implicit declaration of function \
 '_memalign_r' [-Wimplicit-function-declaration]
   35 |   return _memalign_r (_REENT, align, size);
      |          ^~~~~~~~~~~