Before the change UNREG ghc build failed as:
```
  rts_dist_HC rts/dist/build/PrimOps.o
/tmp/ghc2370_0/ghc_4.hc: In function 'stg_newByteArrayzh':

/tmp/ghc2370_0/ghc_4.hc:26:13: error:
     error: 'base_GHCziIOziException_heapOverflow_closure'
         undeclared (first use in this function)
     R1.w = (W_)&base_GHCziIOziException_heapOverflow_closure;
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   |
26 | R1.w = (W_)&base_GHCziIOziException_heapOverflow_closure;
   |             ^
```

It's an UNREG-specific failure because C backend always requires
declarations to be known.

Added missing declaration.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>