small patch to suppress a warning

[Torbjörn Granlund]

I think there might be better places to discuss compiler warnings than
the gmp-bugs list.

I think most of us agree that compiler warnings for valid C code are
sometimes useful.  For example, valid warnings about unitialised
variables seem pretty uncontroversial.

But please continue the discussion on clang's warning chattiness
elsewhere...

-- 
Torbjörn
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