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/last_char_is.c

@@ -19,6 +19,7 @@
  *
  */
 
+#include <string.h>
 #include "libbb.h"
 
 /* Find out if the last character of a string matches the one given Don't