c++: Fix attr-lambda3.C

c++: Fix attr-lambda3.C

Some testcase for a change I was testing had an unnecessary ; at the end of
a line that caused a test failure on the 10 branch, so I mistakenly removed
it from this one.  Oops.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attr-lambda3.C: Add missing ;