check.mak: Fix 'make help' in check directories, it should be 'valgrind.gen-suppressions' not 'valgrind-gen-suppressi...

Original commit message from CVS:
* check.mak:
Fix 'make help' in check directories, it should be
'valgrind.gen-suppressions' not 'valgrind-gen-suppressions'
(not changing target to match help string on purpose to keep
scripts etc. functional).