Fix libssh2 inclusion in libgit2 iOS build script

- Added CMAKE_PREFIX_PATH to arch-specific libssh2 root dir
- Added PKG_CONFIG_USE_CMAKE_PREFIX_PATH so things actually make sense
- Removed unused LIBSSH2_INCLUDE_DIRS

Author: phatblat

script/update_libgit2_ios

@@ -45,9 +45,8 @@ function build_libgit2 ()
     cmake \
         -DCMAKE_C_COMPILER_WORKS:BOOL=ON \
         -DBUILD_SHARED_LIBS:BOOL=OFF \
-        -DCMAKE_LIBRARY_PATH:PATH=../../External/libssh2-ios/lib/ \
-        -DLIBSSH2_INCLUDE_DIRS:PATH=../../External/libssh2-ios/include/libssh2/ \
-        -DCMAKE_LIBRARY_PATH:PATH="${SDKROOT}/usr/lib/" \
+        -DCMAKE_PREFIX_PATH:PATH="${ROOT_PATH}/External/libssh2-ios/bin/${SDKNAME}-${ARCH}.sdk" \
+        -DPKG_CONFIG_USE_CMAKE_PREFIX_PATH:BOOL=ON \
         -DCMAKE_INSTALL_PREFIX:PATH="${INSTALL_PREFIX}/" \
         -DBUILD_CLAR:BOOL=OFF \
         -DTHREADSAFE:BOOL=ON \