Reduce <stdlib.h> namespace pollution.

<stdlib.h> shouldn't drag in all of <string.h>.

Change-Id: I949172f7ff17e329e411e5054e50f79c13775444
diff --git a/libc/include/stdlib.h b/libc/include/stdlib.h
index dfb788e..0060ea3 100644
--- a/libc/include/stdlib.h
+++ b/libc/include/stdlib.h
@@ -31,7 +31,6 @@
 #include <sys/cdefs.h>
 
 #include <stddef.h>
-#include <string.h>
 #include <alloca.h>
 #include <strings.h>
 #include <memory.h>