Be more specific about the `.so` gitignore patterns (GH-17328)

In GH-15823 the pattern was changed from `libpython*.so*` to `*.so*` which
matches a bit too greedily for some packagers.  For instance this trips up
`debian/README.source`.  A more specific pattern fixes this issue.

.gitignore

@@ -6,7 +6,8 @@
 *.iml
 *.o
 *.a
-*.so*
+*.so
+*.so.*
 *.dylib
 *.dll
 *.orig