Undo SONAME bump. CharClass isn't exposed.

Fixes #299.

Change-Id: I00e66da0bb8d245f6f47f8bfaf6eb9637fe97bac
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58270
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/Makefile b/Makefile
index 9dab631..44cedee 100644
--- a/Makefile
+++ b/Makefile
@@ -44,7 +44,7 @@
 
 # ABI version
 # http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html
-SONAME=10
+SONAME=9
 
 # To rebuild the Tables generated by Perl and Python scripts (requires Internet
 # access for Unicode data), uncomment the following line: