linker.h: don't change the soinfo structure

In 48527c3f74, we modified the soinfo
structure to remove an unnecessary variable. However, third party
programs seem to be relying on the structure not changing.
Add a dummy variable back into place to preserve structure.

Bug: 5064106

Change-Id: I3014dda1a46e624644b09e156c3e9381ba3729bd

linker/linker.h

@@ -95,6 +95,8 @@ struct soinfo
     unsigned base;
     unsigned size;
 
+    int unused;  // DO NOT USE, maintained for compatibility.
+
     unsigned *dynamic;
 
     unsigned wrprotect_start;