Add some missing includes to kill warnings when building with the default

Config.h and using gcc's -fno-builtin.  There are probably other files
with the similar problems.
Also, if building against uClibc, don't include asm/unistd.h in syscalls.c
and module_syscalls.c.

libbb/module_syscalls.c

@@ -28,7 +28,9 @@
    _syscall* defined.  */
 #define __LIBRARY__
 #include <sys/syscall.h>
+#ifndef __UCLIBC__
 #include <asm/unistd.h>
+#endif
 #include "libbb.h"