Fix concept checks usage

[François Dumont]

Hi

Here is a fix in the usage of a concept check. There are also many 
testsuite failures when using concept checks but this one forbids the 
library from being built. I know that concept checks are not really 
maintained so maybe the fix is to simply remove those checks.


     * include/ext/random.tcc: Fix usage of _OutputIteratorConcept.

Tested under Linux x86 with and without concept checks.

Ok to commit ?

François

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