Appveyor: Install OpenSSL to resolve a header/library mismatch

[teor2345]

Install the mingw OpenSSL package in Appveyor. This makes sure that
the OpenSSL headers and libraries match in Tor's Appveyor builds.

(This bug was triggered by an Appveyor image update.)

Fixes bug 32449; bugfix on 0.3.5.6-rc.

[coveralls]

Pull Request Test Coverage Report for Build 7159

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1238 unchanged lines in 33 files lost coverage.
• Overall coverage decreased (-1.7%) to 59.088%

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Files with Coverage Reduction	New Missed Lines	%
src/feature/dirclient/dirclient.c	1	18.5%
src/feature/hs/hs_service.c	1	73.47%
src/feature/nodelist/nodelist.c	1	61.26%
src/core/or/circuitlist.c	2	55.45%
src/core/or/policies.c	2	84.6%
src/feature/dircache/consdiffmgr.c	2	90.06%
src/feature/nodelist/microdesc.c	2	73.12%
src/feature/relay/router.c	2	35.02%
src/feature/rend/rendcache.c	2	92.89%
src/lib/container/buffers.c	2	91.2%

Totals
Change from base Build 7077:	-1.7%
Covered Lines:	43158
Relevant Lines:	73040

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💛 - Coveralls