OpenSSL: replace HUNTER_UWP_PLATFORM with CMAKE_SYSTEM_NAME check

[Dargun]

replace HUNTER_UWP_PLATFORM with CMAKE_SYSTEM_NAME to follow cmake style

• I've followed this guide
  step by step carefully. [Yes|No]

• My change will allow compile OPENSSL for windows universal platforms

---

• I've followed this guide
  step by step carefully. [Yes|

---

• I've checked this Git style guide. [Yes|No]
• I've checked this CMake style guide. [Yes|No]
• My change will work with CMake 3.2 (minimum requirement for Hunter). [Yes|No]
• I will try to keep this pull request as small as possible and will try not to mix unrelated features. [Yes|No]

---

[NeroBurner]

looking good. This is the only place where HUNTER_UWP_PLATFORM is used in the codebase. Good thing we move to a more CMake-standard way, thanks a lot

[NeroBurner]

just for future reference WindowsStore is in the official cross compilation documentation in the cmake docs: https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling-for-windows-store

[NeroBurner]

Released: https://github.com/cpp-pm/hunter/releases/tag/v0.24.9