rts/PrimOps.cmm: fix UNREG profiled build

Brought 'CCS_MAIN' into scope, otherwise UNREG
prof build is unhappy:

    rts_dist_HC rts/dist/build/PrimOps.p_o
  /tmp/ghc25593_0/ghc_3.hc: In function 'clB_entry':

  /tmp/ghc25593_0/ghc_3.hc:2983:24: error:
     error: 'CCS_MAIN' undeclared (first use in this function)
     *((P_)(_cly+8)) = (W_)&CCS_MAIN;
                            ^

Signed-off-by: Sergei Trofimovich <siarheit@google.com>