UNREG: include CCS_OVERHEAD to STG

Commit 394231b3 aded
CCS_OVERHEAD annotation to 'rts/Apply.cmm'.

Before the change CCS_OVERHEAD was used only in C code.

The change exports CCS_OVERHEAD to STG.

Fixes UNREG build failure:
  rts_dist_HC rts/dist/build/Apply.p_o
    /tmp/ghc29563_0/ghc_4.hc: In function 'cm_entry':

    /tmp/ghc29563_0/ghc_4.hc:73:13: error:
     error: 'CCS_OVERHEAD' undeclared (first use in this function)
     *((P_)((W_)&CCS_OVERHEAD+72)) = ...
                 ^~~~~~~~~~~~

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

includes/stg/MiscClosures.h

@@ -512,6 +512,7 @@ extern unsigned int RTS_VAR(era);
 extern unsigned int RTS_VAR(entering_PAP);
 extern StgWord      RTS_VAR(CC_LIST);          /* registered CC list */
 extern StgWord      RTS_VAR(CCS_LIST);         /* registered CCS list */
+extern StgWord      CCS_OVERHEAD[];
 extern StgWord      CCS_SYSTEM[];
 extern unsigned int RTS_VAR(CC_ID);            /* global ids */
 extern unsigned int RTS_VAR(CCS_ID);