rts: fix build failure on OpenBSD (_DYNAMIC symbol collision)

Summary:
Build failure on OpenBSD-6.4 the the following:
```
rts/RtsSymbols.c:994:1: error:
     error: redefinition of '_DYNAMIC' as different kind of symbol
    |
994 | RTS_OPENBSD_ONLY_SYMBOLS
    | ^
RTS_OPENBSD_ONLY_SYMBOLS
^
```

On OpenBSD `_DYNAMIC` was always defined in `<elf.h>` headers
but used not to be included.

The change explicitly includes `<elf.h>` as a source of symbol definition.

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

Test Plan: build-tested on OpenBSD-6.4

Reviewers: bgamari, erikd, simonmar

Subscribers: rwbarton, carter

GHC Trac Issues: #15945

Differential Revision: https://phabricator.haskell.org/D5461