Forgot to add compilerplugins/clang/test/unusedvariablecheck.cxx

...to CompilerTest_compilerplugins_clang in
871e420c "Fix loplugin::unusedvariablecheck
check for std classes"

Change-Id: I011d8ff73189dfa85adb4cbd50e556f25be9b79c
Reviewed-on: https://gerrit.libreoffice.org/44592Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

solenv/CompilerTest_compilerplugins_clang.mk

@@ -46,6 +46,7 @@ $(eval $(call gb_CompilerTest_add_exception_objects,compilerplugins_clang, \
     compilerplugins/clang/test/unnecessaryoverride-dtor \
     compilerplugins/clang/test/unnecessaryparen \
     compilerplugins/clang/test/unoany \
+    compilerplugins/clang/test/unusedvariablecheck \
     compilerplugins/clang/test/useuniqueptr \
     compilerplugins/clang/test/vclwidgets \
 ))