RTS: declare setKeepCAFs symbol

Commit 08ba8720 failed to declare the dependency of keepCAFsForGHCi on
the symbol setKeepCAFs in the RTS, which led to undefined symbol errors
on Windows, as exhibited by the testcase frontend001.

Thanks to Moritz Angermann and Ryan Scott for the diagnosis and fix.

Fixes #22961

(cherry picked from commit 42b05e9b)

rts/RtsSymbols.c

@@ -947,6 +947,7 @@ extern char **environ;
       SymI_HasProto(arenaAlloc)                                         \
       SymI_HasProto(arenaFree)                                          \
       SymI_HasProto(rts_clearMemory)                                    \
+      SymI_HasProto(setKeepCAFs)                                        \
       RTS_USER_SIGNALS_SYMBOLS                                          \
       RTS_INTCHAR_SYMBOLS