functional_tests: fix linking on Windows

[iDunk5400]

No description provided.

[moneromooo-monero]

Should this be detected, or is this a "standard" thing ?

[iDunk5400]

One of the commits merged today causes performance_tests linking to fail in MSYS2.

[100%] Linking CXX executable functional_tests.exe
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: CMakeFiles/functional_tests.dir/objects.a(transactions_flow_test.cpp.obj):transactions_flow_test.cpp:(.text+0x1161): undefined reference to `BCryptOpenAlgorithmProvider'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: CMakeFiles/functional_tests.dir/objects.a(transactions_flow_test.cpp.obj):transactions_flow_test.cpp:(.text+0x1184): undefined reference to `BCryptGenRandom'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: CMakeFiles/functional_tests.dir/objects.a(transactions_flow_test.cpp.obj):transactions_flow_test.cpp:(.text+0x11d7): undefined reference to `BCryptCloseAlgorithmProvider'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: CMakeFiles/functional_tests.dir/objects.a(transactions_flow_test.cpp.obj):transactions_flow_test.cpp:(.text+0x12e0): undefined reference to `BCryptCloseAlgorithmProvider'
collect2.exe: error: ld returned 1 exit status

All three undefined references are provided by bcrypt library, according to Microsoft.
https://docs.microsoft.com/en-us/windows/desktop/api/bcrypt/nf-bcrypt-bcryptopenalgorithmprovider
https://docs.microsoft.com/en-us/windows/desktop/api/bcrypt/nf-bcrypt-bcryptclosealgorithmprovider
https://docs.microsoft.com/en-us/windows/desktop/api/bcrypt/nf-bcrypt-bcryptgenrandom
If this can be detected instead, that would be great.

[moneromooo-monero]

Well, maybe someone will come up with that. This patch fixes at least one machine, let's see if it breaks another.

[fluffypony]

Reviewed