rts: export new absentSumFieldError from base

commit b2ff5dde "Fix #15038"
added new stable closure 'absentSumFieldError_closure' to
base package. This closure is used in rts package.

Unfortunately the symbol was not explicitly exported and build
failed on windows as:

```
"inplace/bin/ghc-stage1" -o ...hsc2hs.exe ...
rts/dist/build/libHSrts.a(RtsStartup.o): In function `hs_init_ghc':

rts/RtsStartup.c:272:0: error:
     undefined reference to `base_ControlziExceptionziBase_absentSumFieldError_closure'
    |
272 |     getStablePtr((StgPtr)absentSumFieldError_closure);
    | ^

```

This change adds 'absentSumFieldError_closure' to explicit export
into libHSbase.def.

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