Add OpenSSL lib for libgit2

    -- Could NOT find OpenSSL, try to set the path to OpenSSL root
    CMake Error at src/CMakeLists.txt:180 (MESSAGE):
      Asked for OpenSSL TLS backend, but it wasn't found

Author: techtonik

snap/snapcraft.yaml

@@ -14,6 +14,8 @@ parts:
   libgit2:
     plugin: cmake
     source: https://github.com/libgit2/libgit2/archive/v0.27.0.tar.gz
+    build-packages:
+      - libssl-dev
 
   gitless-cli:
     plugin: python