Remove unnecessary -ldl and clean up includes for malloc tests.

I'm guessing a previous iteration used dlsym to look up the real malloc.

Change-Id: I18be9ef4db4ed059400074c8507d4e2fea882fbc
Reviewed-on: https://boringssl-review.googlesource.com/4100
Reviewed-by: Adam Langley <agl@google.com>