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 ;

gcc/testsuite/g++.dg/ext/attr-lambda3.C

@@ -1,4 +1,4 @@
 // PR c++/90333
 // { dg-do compile { target c++11 } }
 
-auto x = []() __attribute__((always_inline)) -> int { return 0; }
+auto x = []() __attribute__((always_inline)) -> int { return 0; };